Just closed a BofA short sale and we got a quick response (about a week) after we were told they had submitted for approval. The "investor" on that mortgage was Fannie Mae. You should find out who the "investor" is and whether or not a Mortgage Insurance company also must approve the sale. Your agent should contact the listing agent for details.
Now that BofA is under scrutiny by the US Treasury department they are now moving much faster on short sales. There is a new tracking system called Equator that keeps BofA honest. You're very close! Congratulations!