How long a high school student should work in Network in a day?

In this age of computer and internet most of the high school students have craze to work in internet mainly on social networking sites to interact with friends and batch mates.
This will undoubtedly increase their knowledge and communication skills.
But I am worried that some of the students are engaged on social networking sites without completing their home tasks and other routine assignments.

Members are requested to discuss the advantages and disadvantages in this topic mentioning the maximum working time in internet daily by the high school students without hampering their daily study.