On average I'd say I play 2 hours (usually 3 or 4 on the weekend). But I never play when I don't feel like it; I don't make it a chore. I remember hearing Tom Morrello say in an interview that he played strict 8 hour a day practice sessions, and that if he didn't complete it down to the last second he felt guilty, like how a runner feels guilty when he or she doesn't run. I don't think this is a healthy way to look at playing. You need to find inspiration for your playing as well as just practicing your scales, etc.