I guess a combination of access hours using firewall rules and then captive portal vouchers that can let users "buy" lengths of time within that period. That might be a bit difficult to let them have, e.g., 2 hours during that period and that period only. If they are issued with a bunch of CP vouchers then they could go on a splurge and use them all in 1 day. Or if you give them out daily, the user can save them one day and use them the next. Administratively you would have to give them 2 hours worth of vouchers each day which expire at the end of that day…