We set the tv/game limit to 1/2 hour a day (if homework is done) on week days and 2 hours a day on weekends. They can buy more with marbles. This was to give my youngest a positive start (even for her bad days) because she needs some decompression time to veg (she gets that from her parents ). It also helped her keep working at her homework to get that time.
Also, if something is earned (1/2 hour tv for finishing homework), you do not take that away for back talking (at least until the system is working well). Then you add conditions to the "homework being done" like not give dad a hard time, no back talking, no fighting with sis. When we were playing games and having fun time, I would give short explanations that if they made it hard for me, I would be too tired and frustrated to play with them; or, if they fought while I made dinner, I would have to turn off the stove and settle things with them and maybe dinner would be very late. Make them understand that if they have bad behavior, you cannot do those things they want you to do.
at some stage you start taking away the things that make your life easier and more peaceful.
if letting them watch the tv for 1/2 an hour means quiet time for all then go for it.
The trick is to link the tv watching/video games to positive behaviors....like if he does his homework in a set time period, he gets 1/2 (or whatever you choose) of screen time...if he puts his bookbag in its spot all packed for the next day, he gets 'x' amount of screen time....you get the picture....you want to focus on only a few behaviors at a time...give him a lot of chances to do positive behaviors.....once he starts to do the chosen behaviors automatically, then add one or two more to work on...I guess you are right, when the marble is the priveledge, it doesn't really apply to anything else. We can say Yes or No to TV because we're they're parents, instead of because they've "earned the right".... Thanks