Seeing as plenty of people often use a computer that much or longer on a regular basis (not counting breaks to eat or go to the bathroom, obviously), I'm not really convinced it's just the how long you use a monitor. I'd suggest talking to an ophthalmologist for real advice.

That said, I think the usual line is to take a ten minute break every hour. Should be good for your legs, too. But really, talk to an ophthalmologist.