https://addons.mozilla.org/en-us/firefox/addon/leechblock/ does windows of access, allows for x minutes, has grouping. Course, it's FF only where hosts file gets your whole connection.
latest version has a neat feature which tracks links from blocked pages. Ie if I give myself 15 mins of HN a day, but end up wasting 2 hours because I only spend 10 secs on HN opening up tabs to read, it now tracks that. Any links you click from HN will count towards your time limit.