I'm far more lax in the summer than during the school year, but during the school year the rules for DD7 and DS10 are these (I need to preface this with the info that we homeschool, so homework in the evening isn't really an issue): no computer or TV during the week unless the family is watching something specific together. Each day they complete standing chores by 5 p.m. to earn one mom buck. On Friday after school they can exchange mom bucks for a rationed amount of candy or for computer time (1 buck = 20 minutes for a maximum of 1h40), plus they get 2 hours of computer time for free to use at their discretion over the weekend. Over the summer, additional time to be used during the week can be earned by doing more involved / less pleasant chores, like weeding or cleaning the bathrooms. If any character issues or academic issues arise we dock the free weekend time (ie, if general responsibilities of family membership aren't met, optional family privileges are revoked).
This system has worked pretty well for us. The expectations are clear in advance and can be tweaked. The drawback is that my son can be a little bit mercenary, but it does draw a clear picture of working for privileges, and that the work has to come before the paycheck. It's likely that I will be reducing the "free" weekend time this year as my son will go to a magnet environmental school with a heavy workload (not that our homeschool wasn't heavy; there just wasn't all the time-sucking classroom stuff!).
HTH,
KDV
