With my ds5, I set a timer. When the timer goes off, the computer goes off (I do give a 5 minute warning). Also, my household flows better with a schedule, so ds knows the routine. But that's how it works for me. There have been times when he's really into something and can have more time. I think you have to figure out what you are comfortable with.
We use computer time as a reward, so if he doesn't cooperate when it's time to transition (within reason), no computer time tomorrow. As much as I love all that he can learn on the computer, he needs to do other things too.
