how long? - virgins or mated? former obviously not for long. the latter for as long as necessary (see below)
how many? - perhaps this might help!:
http://www.beesource.com/files/21bankingframe.jpg
accompanying text from beesource:
"Here’s a picture of a frame that can hold 60 queens. The hive is prepared by putting the queen below an excluder, then some frames of young larvae are put up above the excluder, and the banking frame is put between the brood frames. When i was doing this commercially we could store up to around 180 queens in a queen bank hive, although less is better. The hive was maintained weekly by lifting fresh frames of eggs and young larvae up around the banking frames to keep a good supply of nurse bees there. They feed the queens through the wire and queens will often continue to lay, the cages will sometimes have a pile of eggs in the bottom.
The frame is built to fit in a standard lang with correct bee space (same as a normal frame but wider). 3 shelves works best for good cage size. The cages were made from strips of timber 1 inch wide, 1/4 inch thick, and 1 7/8 inch long. Just cut timber to those lengths, assemble as per the pic and cut the door and the mesh to fit. Leave a little bit of wiggle room on the door but not too much.
Queen banking attracts controversy because some people believe keeping the queen locked up for a couple of months must have a bad effect on her, although it has never been proved one way or the other. However it sometimes has to be done if there just isn’t anywhere else for the breeder to put his spare queens temporarily.
Other literature on queen banking talks about losses, and sometimes recommends keeping quite a low number of banked queens in a hive. However I don’t remember ever losing a queen in a bank, I think it’s just about having a healthy strong hive, and not asking more of them than what they can do."
