I'm no expert, but from what I've read you should awaken without feeling groggy or tired. I've personally had the luxury of being able to sleep according to my own schedule (without waking times or alarms, simply sleeping when I was tired). For myself to reach that well-rested feeling, I should give myself 9 hours a night. Sometimes I need 7, but sometimes I need the 9 hours. I very occasionally need more, and very rarely sleep less (though it happens).

What I know affects how much sleep I need or how long I should give myself to sleep varies. I'm female and follow a cycle closely linked to hormones as a baseline. If I wake up at night (for toilet, etc), I need a bit more time. If I increase physical activity, I have an adjustment period that requires more sleep.

I'll also note that time of sleep impacts me. I'm better off waking after 9am (naturally this is 11am or so). I cannot always get enough quality sleep waking at 6am and 7am leaves me slightly deprived. I tried keeping a schedule even on the weekends, but I felt horrible and found it better to sleep a bit more to catch up.