How to limit access time to the Internet via Wi-Fi?

1) using iptables all traffic driven to your http server, prompting you to enter a password
2) after entering the password added to the front of the iptables redirect rule, the traffic from this Mac address to drive in the intranet using snat
3) after the lapse of half an hour deleting the rule.

To set a proxy server script to generate the password and add the user to the allowed list. Another script to regularly check how much time has live password and clean if necessary.
From the equipment, not the computer. Of distribution of *nix and squid. If there are no experts on *nix, it is possible and on the distribution of Windows to spend.

If You have a place, and do not want to fool with software solution, it is possible to look in the direction of specialized hardware, like D-link DSA-3110 hotspot edition. Surely there are similar from other manufacturers. Of the benefits — it is easier to configure and maintain reliability when used by unskilled staff (cashier/waiter) above and any possible big price in comparison with the variant "old PC + Linux" will still pay off when use it in public.