I want that one of the users (not in the sudoers list) have Internet access from 14:00 until 16:00 and from 17:00 until 18:30. Is it possible? Can I set different time intervals for different week days?
You can use iptables's
Now you can use cron to add or remove those rules (which may need a little bit of shell scripting if you already have some iptable rules or want it to to for different users at different times).
tested on Ubuntu 11.10 (oneiric), locale: he