When I have the chance to get a good, natural night's sleep - that is one without any outside interuptions and without any medication to help put me down - I tend to go around 7 hours.
Usually, if I stray from my normal 7 hours, it is on the lower side. By that, I mean I am more likely to sleep for 6 or less hours than I am to sleep for 8 plus hours. I guess it has something to do with the fact that I never really enjoy staying in a bed at all unless I am really hurting and need a rest. Usually around school time, or when I have something planned, I will fall into the less than 7 hours average due to having a hard time staying asleep. I only really go over 7 hours in I am taking sickness medication. I do know that if I go to sleep, and nothing wakes me up before my body naturally wants to (as in no noises or movement) I will stay asleep for 7 hours after I initally fall asleep.