Blair, you can make them up a week ahead of time if you wish as long as you have brood in there to keep their laying worker "gene" suppressed. However as Iddee pointed out, they will start trying to make their own queen like in a walkaway split. The longer a hive is queenless, the harder it is to get them to accept a "foreign" queen. If they have their own cells started they may kill yours and wait on theirs.