9 hours is my sweet spot. Anything less than that, and I don't think as well. Below 8 and I'm generally a bit of a zombie, and operating at 50% speed. Above 10 and I tend to get groggy and depressed.
I ask because during times when I get regular strenuous exercise this number comes down to about 7.5-8 hours.
I do get really deep sleep though. I would really prefer if I could sleep longer but my job shifts prevent that.
9 -10.5 hours normally, 7-9 hours if I've been working out routinely.