My kids still at home are 14, 14 and 10. We do not permit any screen or electronics time during the school week. This means no computer games, no TV, no PS2 or Wii, etc. They can use the computer for schoolwork and e-mail as necessary. They don't have access to IM, MySpace or Facebook at all.
On Fridays through Sundays, I don't put limits on electronics, provided they get all their schoolwork done. I might get a little twitchy if they've been in front of a screen for hours
but I might not.
I found that when I allowed electronics during the week, the kids were less likely to want to participate in extra-curriculars. Now that screen time isn't a weekly option, they are excited about joining in clubs (son is in Latin club, improv and the satire newspaper; daughter is in an honors club, plus volleyball; other daughter is in drama club and gymnastics).
It works for us.