I have these things in placed called "time limits". They require me too get off at a certain "time". The times are,
Weekdays (when it's school): When I get home which is usually 3:15 pm - 6pm.
Weekends (both school and holiday): After 1pm - 6pm.
Weekdays (holidays): After 1pm - 6pm.
I usually do much other things beside the fact I'd rather do more things on the computer than socialize with the kids of Australia where I live.
I don't have time limits, but I kinda have a schedule for how long I spend on the computer. I don't know how long I spend exactly, but probably around 2-3(+) hours a day, only because I have nothing to do in that period of time.