20+ hours, at least. Longer if you really dig into the side quests, which are many, and have many ways to resolve (Arguably too many).
2-Story ?
The story is interesting and involved, with many twists and turns and revelations. It can sometimes be a bit predictable, but it's still a joy to go through.
3-Impressiveness ?
It's impressive.
4-is it worth $45 ?
...No.
5-how does it compare to Planescape ?
It's been so long since I played it, I can't be sure. T:ToN is definately more technically impressive, and it has all the modern gameplay updates one would expect, but relative to era... it's not as good. Planescape: Torment was a true gem of it's age, while Tides of Numenera, quite frankly, gets shown up in almost every regard by Tyranny, a superior game.
Without going into it in to much length, T:ToN's biggest strength and weakness is it's world. The world is massive and highly detailed, which is normally good, but it's so dense that there's just too much going on to focus on. 90% of it's going to get lost in the colossal shuffle that is the story, and that includes companion characterization.
It's second greatest strength (Which, because it's all good, I guess makes it it's actual biggest strength?) is it's resolution chains - Failure can be good, and that includes dying. This helps by reducing save scumming, and keeping the game moving forward, rather then forcing you to try the same thing over and over again until you get the result you like.
Everything else about the game is, at best, mediocre compared to the genre as a whole, or a failure. Overall, I'd say it's a decent game with a lot of holes in it.