As my free time dries up, I'm starting to prefer short titles. There's still the occasional, mindlessly grindy type of game that I enjoy having on a handheld for my downtime, but for the most part I prefer short, well tuned experiences. One of my favorite games last year was Gone Home, which ran me $20 and only lasted me about 3ish~ hours, but touched me quite a bit. I consider it money well spent.
Loosely though, if I spend $10-15 on a game, I'd at least like to get 4 to 5 hours out of it - if it's not the kind of game with lasting replayability. Maybe $5 bucks for a game that I get a fun 2 hours out of.