Probably 6-7 on the week before a test, 10 or so the weekend, then 8-9 the week of the test with 12-14 the day before the test.
We had 2 week cycles btw.
What will confound this question is that some people's 8 hours is other people's 11 hours. I had a friend in college that was always at the library because I think it made him feel like he was really putting in the time, but honestly, if you tracked the hours that he studied versus chatting with friends, perusing the internet, sleeping--I don't think he put in any more time than I did, but he was at the library a lot more.
I think volume is important, but more important is quality. I like the suggested 11-hour schedule that I saw somewhere: 8-12 study, break 12-1, 1-5 study, 5-8 break, 8-11 do questions, sleep at midnight. I think it breaks up the day and gives you some good study time in the early part of the day when you're fresh, and relaxation in the later afternoon when you need it. But the key is actually studying from 8-12, not studying a little bit, checking SDN a few times, reading a couple articles out of the NYTimes, boiling some tea, check the Cavs score.
Is this serious? You can't take a 5 minute break during your day to boil some tea that will probably make your next hours of studying a bit better? Maybe if you feel a bowel movement coming on you should hold that too, because you wouldn't want to interrupt your study time.