How many hours do you leave your child on Internet??

I have four children, 19, 10 and two 8 year olds. Only thr 10 year old is really interested in going on the internet. He isn't allowed on before school, but has a slot when he gets home from school for 15 mins or so and maybe two more slots of 15 mins before he goes to bed. Weekends he does get a few more 15 mins sessions during the course of the day. He isn't allowed to do much on the interenet as we are very careful with the net