To me, it depends on the kind of game. An RPG can be 30 hours long and that's fine, but if it lasts 50 hours + I won't buy it.
Generally, 10-20 hours of play time is ideal for me. The games I recently played are Metroid Prime (8 hours), Metroid Prime 2 (10 hours), Walking Dead episodes one and two (2-3 hours each) and God of War: Ascension (10 hours). I sometimes buy games that last very long, but I never finish them: I don't have much time for gaming these days and I want some variety in my roughly 5 hours per week of video game time. Playing a single game for two months straight isn't much fun.
There is definitely a market for shorter (and cheaper) games. I won't buy Witcher 3, Xenoblade X or Final Fantasy for the sole reason that I'll never finish them. My friends are heavily into DOTA 2 and I was interested - until they told me "you get really good after 500 hours". If a dev says "The game has 100 hours of playtime" I run for the hills. That's an absolute deal breaker to me.