M.C.
My DD (23 mos) is allowed one episode of Dora per day--about 20 minutes (she could watch something else, instead, but she always picks Dora). Some days she doesn't watch any, depending upon what we're doing. But we don't have a TV in the living room--the only one is in my husband's office/den, at the back of the house, and my husband and I don't watch much (we actually don't have TV--no cable, no antenna--we only watch DVD's). So out of sight, out of mind, helps, plus she doesn't think of it as something that other people do and she doesn't--she pretty much NEVER sees me or DH watching TV, since we'll just watch an occasional movie after she goes to bed.
When I was a nanny with slightly older kids (4-8) the rule was no TV at all on school days, then movie night on Fridays, and no strict rule on Saturdays or Sundays, although we were usually too busy doing other things to watch much.