Remember when I first had Christopher and I promised he wouldn't watch TV until he was five? That lasted until Hannah came along 21 months later. First there was Sprout and Disney Jr. Then we got a Wii and recently the kids have discovered Minecraft. Oy.
About halfway through Christmas break I decided that the kids had been having too much screen time and told them to turn it off. There was much wailing and gnashing of teeth. When they calmed down they asked what they could do to earn it back and the 45/45 plan was hatched.
On weekends and days off from school they can have 45 minutes of screen time and then the next 45 minutes belong to me. During my time I have them do something helpful, which usually takes about 20 minutes, and then they can (have to?) read. After that they get another 45 minutes of screen time. They can play during their break, with each other or by themselves, but it doesn't count towards the 45 minutes that belong to me.
It's been working out fairly well. The kids have had lots of book time, my house is pretty darn clean and they can build their Minecraft universe to their little hearts delight... in 45 minute blocks of time.