How long (as in days) can you keep a package of bees in a cage before installing them?
If absolutely necessary, for a couple of weeks as long as you refill the feeder and/or spray them down with syrup once or twice a day and keep in a cool dark place.
Great antidote Brian has layed out, just to emphasize,
cool dark place, as it gets downright hot out this becomes extremely crucial as they can get overheated and die off.
...JP