When a match on which you have bet is postponed, the bookmakers offer the odd 1.00 for that pick. So, if it is the only event on the slip, you will receive the stake back and if the event is placed on a slip together with other picks, then the postponed one will no longer be taken into consideration.
Bookmakers have different periods in which they offer the 1.00 odd for postponed matches. So, at some you will have to wait 24-48 hours before the bet to be voided and at other it can be done faster. It all depends on the official information showed on the websites of the federations that organize the event.