We used to allot 15 minutes of computer time a day for our DS4 and DS6. But then we started making them earn their computer time and it's worked out great!
They each have a jar, and during the course of the day, they can earn 1 minute, 5 minutes or 10 minutes chips (poker chips). They earn chips by being kind to one another (playing nicely for a period of time together, DS6 reading books to DS4, etc.) and other behaviors that need to be modified. This shifts over time.
They still can have only one computer time each day, but they can choose when to do it. Then if they earn more chips that day, they carry over to the next day.
We also make exceptions at times when they are doing something educational on the computer. For example, during the summer my DS6 used Time4Learning. Sometimes he spent hours on there, but he was learning a lot, so I was fine with it.
