Firewalla can block/allow activities based on a schedule, say from 10 am to 1 pm.
What this is asking is to aggregate the time spent on certain activities (e.g. gaming), and set a limit for it over a certain period (e.g. throughout a day or week). Firewalla can not do this accurately, and the margin of error is very high.
The technology behind this involves predicting people's behavior at the network level, and it is something very difficult to do accurately at the moment.
Update: it is very likely firewalla will be supporting this kind of limit to a selected set of applications in app versions 1.60 and 1.61