I think children and adults alike should be encouraged to lead a balanced life. I confess it is easier to sit in front of a TV and just wile away the hours; I am guilty of it myself.
Perhaps the question is not how much is too much but rather is there enough of other things in life - exercise, social interaction, family.
Having said that, there are times when TV, can be a comfort, perhaps to the ill or elderly who have little else to occupy their time.