Just run a timer that calls an alias every X amount of time. When the timer triggers, loop through the users in the channel using $nick($chan,N) and /inc their variable or hash table data or however you choose to store their tokens. At the same time, you can delete anyone in the list who isn't on the channel anymore. If you want to allow them to be off the channel for more than just one timer interval, then store the $ctime when you last incremented their token. Once that's stored, you can subtract it from the current $ctime and if it's greater than your limit, you delete them.
