0-3 years old children: Keep children of this age away from the screen. This should include all the computer, tablet, and smartphones.

3-10 years old children: Spending 1-2 hours in front of a screen can be found reasonable, provided that they are under control.
Children older than 10 years old and teenagers: Spending 2-4 hours in a computer can be found reasonable, provided that they have completed their home assignments, helped for the home labors and fulfill their responsibilities. But, it is essential that they fulfill their essential daily responsibilities.