Depends entirely on the game...I spent a good 6 weeks or so on Songs of Conquest, then finished Airborne Kingdom in a couple days.

When you defeat Earth in the final epic battle, OBVIOUSLY.

Depends on the game, but as already mentioned, free time is the biggest issue.
2 hours a day quality gaming time is nearly the max I can get.

I started the gaming hobby late, in my early 20s, so (thankfully) I’ve still got enough energy to play. Another thing to keep me focused is that, excluding the first 4-5 years, I play only one game at a time, until completion. No different games, different stories/lores, different controls, etc., at the same time. To me it works better. Of course it can happen to be tired with a game (e.g. long RPGs). In such cases I stop it and I play (until completion again) a shorter game (adventure, action, etc.) and then I return to the stopped game. I usually have 2 h available on each day, which is a good amount if you stay focused to that one game. I think that if one tries to split his 2-3 daily hours in 2+ games, he would get nowhere.

Aside from "time factor" and "real life" (but that's very trivial and obvious thing, so I'll skip it), I think a lot depends on the type of the game and challenges it presents.

I'm seeing the trend that if I play a game requiring more "wits" and "know-how", then no matter if I'm playing game I was playing back then (but didn't finish it) or playing something completely new -I'm actually surprised how well and "fast" I'm doing with such a game until the very end. Some examples - Fallout 1 and 2, Fallout Tactics, Telepath Tactics, Black Moon Chronicles.

The Adventure Games and Puzzle games are probably the most "extreme" in that matter, as in those two genres "know-how" is basically an entire game.

But the games that are more oriented on "action" part, are a mixed bag (by "action" I mean veery wide range - First/Third Person Shooters, Platform Games, Racing games etc). It depends on how it's made, how much is the "random" factor involved and how many there are "arbitrary" solutions, that makes playing through it harder.

Two examples of platform games, to illustrate this:

1.Leo the Lion - I've finished the whole game in my teenage period, but I would probably have a lot of difficulties doing it now, due to many "obstacles" such as "which element is background and which is the platform/solid thing"? Or "which way should I go - is jumping here even possible, or just very hard due to the character's control? Not to mention about infamous "leap of faiths" in platform games in general.

2.Jazz Jackrabbit 2 - it has very limited those "arbitrary" limitations, so going through it (on hard) was for me surprisingly easy - but it's due to the fact that it's in general game with "fair play" rules, you can see everything on time, and if you make a mistake, it's on you, not on the control or unclear level design.

TLDR - when games engages mostly intellectual capabilities, I found out that I'm making through games much better than when I was younger.

For action-oriented genres - it's a mixed bag, depending on how "fair" is game and if it's necessary to memorize the level etc.

All the comments here sum things up pretty well. A mix of lack of time, motivation, feeling "amazed" by games, game type etc. It's just not as easy to get lost in a game.

That is very, very true! As an aside, to the extent that I do feel wonder these days, it comes from mother nature.

As for the question, it takes me a few weeks to finish a game [and sometimes months for RPGs]. Even if I've completed a game though, I don't consider that I'm 'through' with it until I've exhausted all possible outcomes/discovered everything. I tend to play and re-play games until I feel as though I've scratched that itch.
Of course, if it's a dud game, I will be 'through' with it in minutes! :D

Time isn't such an issue for me. I'd actually say I'm much more free in this regard now as an adult than when I was as a teenager under surveillance of my parents and school and homework taking up so much of my day. Of course I have quite a bit of work now, too, but my schedules aren't as rigid as they were back then. (Though it certainly would be different if I had a regular 9 to 5 job and/or kids.)

For me, I guess what slows me down is mostly the overwhelming supply/access to games. For one, I play a lot of games at the same time, and then I also tend to buy so many new ones in order to sample them that for every finished game, there are x new ones added to my list.

I think these days I'm also a lot more bothered though if I feel games are wasting my time with filler content and grinding, which makes me drop many without ever finishing them, in favor of the next games, because I'm not forced to stick to any of them anymore (compared to when I barely had games at all).

LIMBO: 1 day.
Portal: 2 days.
Baldur's Gate: 5 years.

So there is some variance how fast I finish games these days...

You are right that back when I was young, single, childless etc., I tended to concentrate more on games. With kids and wife around etc. there are more distractions now, so it is too much that I plan to play some game "this evening after kids go to bed" or "next weekend", but then when I am supposed to do that, I either just hang around in Youtube or GOG forums, or just feel sleepy and decide to hit the sack.

Also it doesn't help that I still feel much easier to just jump play a quick 30-60 minute match of an online shooter (Team Fortress 2), instead of playing some single-player game that doesn't necessarily have save-anywhere, so I can't drop such game at any time either without losing my progress.

So, it is about there being more distractions nowadays, both due to wealth of gaming and entertainment options, and also living a busier time in my life.

I keep thinking that when my kids grow up a bit and I reach retirement age, I have again plenty of time to concentrate to games and then I will finish all the thousands of games I've meant to play for decades. But with my luck, at that point I instantly get pancreatic cancer and ALS and die off, so I can forget about playing any of my games. I feel that I actually had the most free time when I entered work life (after studies), but were still single. Then all my evenings and weekends were free by default, while e.g. in the high school, university etc. I often had to study or do homework in the evenings, weekends etc...

Plus when I was at the university, I also worked a bit on the side to get some extra money, a couple of evenings + the weekends every week, so yeah I didn't feel I had much of free time. And no long summer vacations either as I worked those months of course at the same place, just to get some extra money.

Damn I don't miss the student life at all. I was poor and still not much free time, it sucked in every way, I would probably call it the worst and most stressful time of my life. Getting my first real job after studies was a bliss from the start, I had both money and much more free time too. Getting married changed that again. but hey at least there is less stress now about dying alone at my keyboard while playing Team Fortress 2, as a single.

I presume "something" means either our cognitive abilities, reflexes, eye-hand coordination etc. which are prone to deteriorate with time, which should affect our ability to play and finish many types of games, especially fast action games.

I recall thinking as a kid that when I am middle-aged, I probably am so old I am unable to play any games, keep losing all the time etc. To my surprise, I haven't seen much of that so far, and quite often actually when I play some old game now that I found very hard as a young boy, might actually feel surprisingly easy now. I still seem to easily beat other players age of my sons in e.g. Team Fortress 2.

I don't mean to say my cognitive skills, reflexes etc. must have improved with age, but they haven't deteriorated the way I feared as younger. Maybe I have still become a better gamer overall through mere practice over and over again, I may have learned to compensate e.g. waning reflexes or whatever by doing things differently in games (hiding my inability to play the game as I used to) etc.

Still, I don't remember examples of being a wizard is some game as a kid or youngster and now being unable to play that same game. Maybe it is because I sucked in Super Street Fighter 2 Turbo also as a kid and just didn't want to learn the game's complicated controls, so it hides the fact that nowadays I may suck even more in that game.

Naturally, if I suddenly contract some nervous system disease like Alzheimer's, ALS, Parkinson's etc., then yeah I guess my ability to play games goes south fast. Hasn't happened so far. What has affected my gaming maybe the most so far is vision, which means reading small text from a small screen is a no no, at least without good reading glasses. Hence I am not a big fan of handhelds or even laptops with smaller than 15.6" screens (and even them I prefer playing on a bigger monitor).

As we get older, we are no longer as fresh ... or some might say, simple minded or very focused with a sort of tunnel vision.

As the years roll by, you get more things on your mind, more distractions, more responsibilities, less time, and often the reward from a game is no longer what it used to be ... a bit like sex for many, probably most.

Variety can of course, help you keep things a bit fresher, as can significant breaks.

Mindset no doubt plays a part. So your overall approach to a game or gaming.
Are you in a hurry, or do you want to savor, etc.
How many games are in your backlog, that you are hanging out to get to.
What are your reasons for gaming, etc ... and playing the types of games you play, especially if that has changed.