Well, answer is too short “A lot.”
As for me, I sit at the computer 24 hours -9 hours=15 hours. Probably there are people who do more and for those who do and probably for me too, I will review several situations when one should turn the computer off, according to the Jokes provided by the “Welcome to Computer Jokes”

These are:

1. A friend calls and says, "How are you? Your phones have been busy for a year!"
2. You forgot how to work the TV remote control.
3. You see something funny and scream, "LOL, LOL."
4. You meet the mailman at the curb and swear he said YOU'VE GOT MAIL.
5. You sign off and your screen says you were on for 3 days and 45 minutes.
6. You fall asleep, but instead of dreams you get IMs.
7. You buy a laptop and a cell phone so you can have AOL in your car.
8. Tech support calls YOU for help.
9. You beg your friends to get an account so you can "hang out."
10. You get a second phone line just to call out for pizza.
11. You purchase a vanity car license plate with your screen name on it.
12. You say "he he he he" or "heh heh heh" instead of laughing.
13. You say "SCROLL UP" when someone asks what it was you said.
14. You sneak away to your computer when everyone goes to sleep.
15. You talk on the phone with the same person you are sending an instant message to.
16. You look at an annoying person off-line and wish that you had your ignore button handy.
17. You start to experience "withdrawal" after not being online for a while.
18. "Where did the time go?"
19. You sit on AOL for 6 hours for that certain special person to sign on.
20. You get up in the morning and go online before getting your coffee.
21. .....You end your sentences with.....three or more periods.......
22. You've gone to an unstaffed AOL room to give tech support.
23. You think faster than the computer.
24. You enter a room and get greeted by 25 people with {{{hugs}}} and **kisses**.
25. Being called a "newbie" is a major insult to you.
26. You're on the phone and say BRB.
27. Your teacher or boss recommends a drug test for the blood shot eyes.
28. Your answering machine/voice mail sounds a little like this...."BRB. leave your S/N and I^ll TTYL ASAP".
29. You need to be pried from your computer by the Jaws-of-Life.

If you identify several reasons from these 29, Please turn the computer asap. ;-)

But before turning the computer off, you should know top ten reasons why computers are male.
10. They have a lot of data but are still clueless.
9. A better model is always just around the corner.
8. They look nice and shiny, until you bring them home.
7. It is always necessary to have a backup.
6. They'll do whatever you say, if you push the right buttons.
5. The best part of having either one is the games you can play.
4. In order to get their attention, you have to turn them on.
3. The lights are on but nobody's home.
2. Big power surges knock them out for the night.
1. Size does matter.

Does anybody disagree?