I like to think of games in terms of number of hours. The longer the better, with about 10 hours being very minimum. If you charge for $20, 3 times a movie where I'm from, you'd better make the game last 3 times as long
I like games that take me at least 25 hours of good play. Some say games can be too long, but I think the real issue there is too much time and too little dynamics. If it keeps changing and getting more interesting as you play longer, then it's like playing a new game that you're already good at every day.