It can take 48 hours or 2 weeks. Generally (not all the time), when a short sale is "preapproved," it usually means that Bank of America has given their "net proceeds number" - meaning the money amount they need to net from the sale of the property. A HUD statement needs to be run (the list agent takes care of this) and supplied to Bank of America along with the offer to purchase in order to receive the actual approval letter. The approval letter is very specific with terms and conditions and a definite closing date - it also includes wiring instructions. Even after the seller receives this letter, BofA still needs to "approve" the final HUD statement right before close -- which sometimes goes very quickly and other times, they make numerous verbiage changes that only delay the closing process.
Always remember though - whenever you are dealing with a bank ... they literally have the control and you are basically "at their mercy." But patience and persistance does pay off!
Hope that answered your question and good luck with your purchase!