It doesn't make sense to choose an arbitrary number, the central question is: Is it ruining my life? If some is on the Internet for every minute of their spare time, and the overall quality of their life is greater, or about the same as before the Internet, then no medical illness is present. However, if another person is failing to go to school or work, losing important relationships, and suffering other serious negative consequences, AND they cannot stop in spite of all the loss, that is probably an illness.
Illness doesn't mean excessive use, it means very serious consequences, up to and including death. This is an uncommon illness, but probably not rare.