20 hours is a bit much for a game I played before... I prefer a game that I've played before to take me ~15 hours to beat (somewhere near 90+%).
But for games that I haven't played before, I like it when they're about 40-50hrs. But then again, I think I played Luigi's Mansion for the first time like a week ago. Took me 8 hours to beat (I took breaks and stuff, I'm a pretty busy guy) and I had FUN.
On the other hand, I'd gladly play 100+ hours for the newest Metroid, Golden Sun, Zelda, Mario, etc game.
In a wrap, I like games that can captivate me for awhile, but come time I want to replay them, I want to be able to enjoy them in one sitting, from start to finish, like a good book. (Hence, 20 hours of gameplay is pretty good when I'm on break. Just sacrifice a day of my life for sheer joy)