How much do I need on an average day?
4 Hours or less: Outside the bounds of what I'd consider an average day. Experience suggests that I can function, but most of my awareness is internalized. The world seems surreal, and I'm apt to ignore things while I ponder the mysteries of color and sound.
5 Hours: Not enough. I can function, but I'm cranky. Reaction time is terrible, thoughts are less fluid, awareness is sub par, and I get clumsy. I reach a moderately acceptable level of performance later in the day, but getting motivated about anything is difficult.
6 Hours: Adequate. It takes me a little time to reach a normal state, but I get there while it's still morning. Suitable for day-to-day activities.
7 Hours: Good. This qualifies as sleeping in, since I'm not likely to still be sleeping after seven hours on an average day. Barring illness or injury, I awake refreshed.
8 Hours or more: Outside the bounds of what I'd consider an average day. It happens sometimes, most likely following a night of much less sleep. I awake refreshed, but no more than after about 7 hours. Exceptionally long periods of sleep (10+ hours), on the few occasions they've come up, tend to leave me groggy.
On a typical night, I get around 6-7 hours of sleep, so that seems about right.
As an interesting observation, the rapidity of my "waking up" is directly proportional to the information flow of my environment. Plugging into my newsfeeds (browser/e-mail) or being in an active social setting (Festival mornings) gets me going in relatively short order.
21st Century Comment: Newspapers don't seem to have a very good navigation system, their search is slow, and they have far too many ads.