There are no rules because betting isn't supported by the staff. Obviously welching isn't allowed and people who don't pay get locked, but there aren't any time limits or regulations regarding payouts.
Generally, if a bet is confirmed, you wait for the person to be online, PM him to remind him that he has to pay, and if he doesn't reply to you in a day or so, you report him. If he doesn't go online for several days, then you can also report it.
It's generally pretty much common sense, and it's better to try and settle it through PM's before you make a formal accusation.