Hmmm, I think it varies from child to child. Time on TV = time away from learning. But my boy only gets to watch TV when he eats (definitely can't do much learning activities during that time, so might as well let him watch educational DVDs).
I dont't find my son's language abilities being hampered by TV. In fact, he learns alot of vocab & expressions from there (that's why it's only limited to EDUCATIONAL programs, no violent cartoons).
His attention span is the same as any other kiddo. If the lesson is interesting, he'll be quietly seated down & following instructions. If the teacher is boring, I won't expect him to sit still either.
It depends on how parents us TV as a learning tool TV is NOT your babysitter