My DSs don't have to "earn" tv or computer time, as long as they have all of their homework and chores done, they are allowed use of their tv's and computers.
They can, however, lose their tv and/or computer access due to bad behaviour, or if we find out they didn't hand in a homework assignment. This isn't usually a concern in our house since they are both honor roll students and conscientous workers, but it has had to be implemented once or twice in the past.