I wish there were a 'blanket' answer to this one. However, when the bank is involved, this will vary. It depends on many factors, such as how skilled is the bank's negotiator? How many people does this pass back and forth through prior to going to the bank, etc., etc.? I have buyers who have been waiting on bank approval from a bank for a 'short sale'. We wrote a strong offer in January - and we are STILL waiting. As the buyer's agent, my hands are tied. The listing agent (for the sellers in 'short sale,') is the only one permitted to contact the bank's negotiator, and that's been less than amazingly slow, as that negotiator is quite lax on returning phone calls to the agent...SO - we wait...
I wish there were better news...sometimes this CAN go quickly...but it depends largely on the individual bank's policy/procedure in handling short sales and/foreclosures.
Best of luck to you!