Yesterday, I closed on a short sale that began on Feb 16, 2012.
If time is a factor, there may be trouble brewing in the kingdom. The investor may choose to do a shakedown of the exiting owner or the present buyer, accept the offer or decline it completely. And they will do so in accordance to the schedule kept by the investor board meeting.
The only reliable answer to 'How long?" is "Too long."
The best guidance is to keep this clinical. Don't allow your emotions to get involved. The seeds of impatience grow the fruit of disappointment. Let the process do what it will and don't get married to the result. It can be heart breaking.