I intend to play this series in order. Don't try to stop me. But I'm just curious about how many hours it will take to finish each game. Different estimates from different people are welcome.

With a walkthrough or not ?
The first one without walkthrough : 100 hours or more. A LOT of mapping to do (At least 50 dungeons if memory serves).
With walkthrough and maps : About 20 - 25 hours I think.
Second one : Same thing.
After, I don't know until the 6th : about 20 or 25 hours without walkthrough.

MM1: 120 hours
MM2: 250 hours
MM3: 100 hours
WoX: 200 hours
MM6: 200 hours

The first one is 55 16x16 map grids, 20 of which are linked together as the over-world, 5 towns, 9 caves, 6 castles and 15 "other areas" such as dungeons and other dimensions.

That's about $1 per hundred hours...

Swords of xeen : 50 hours, until you encounter some game breaking bug or you curse yourself for not saving up on the regular weapons

I've only beat MM2 & 6:
MM2 (Sega Genesis): I spent at least 500 hours playing it (got my chars to lvl255)
MM6 (PC): I probably spent ~250 hours on it - didn't max out chars, but if I remember correctly, I came fairly close.
Note: I don't play games to 'beat' them - I try to experience as much of the game as possible.

I can't look at this without laughing. I mean that in the nicest possible way. I want to see this on a GOG banner ad.
Out of curiosity, how's it coming along?

I don't know about him, but I'm doing the same thing, and I only have my characters up to level 6-7 in MM1.

Regular weapons? As opposed to what, what do you mean? (I have never played it yet so I haver no idea)

magical weapons or magic. The elder items you get in the game: keep them. you'll need them, but only at the end of the game. oh yeah, and the enemies there rape (with a big stick with nails in it covered with sandpaper) your party with their energy attacks, which totally ignore your fancy pancy full obsidian armor and underwear.

Sadly, RL is leaving little time for games, lately. I still intend to play this series in order. Eventually. But I've also been intending to play Planescape Torment since around 2002.