Typically, I will ask the listing agent if they have worked with a particular bank before, regarding time frame and ease. Some of the banks get their stuff done quickly, where others just seem to go about the day-to-day aspects of their business.
When a bank is large, like Wells Fargo, you may have to do a little digging to find out who the actual contact person is regarding your short sale. It may be that they are also engaged in checking out the "clear" title...finding any leans and such on the current property, prior to going forward with an accepted price from a buyer.
Four to six weeks would be a conservative estimate.
Good luck with it. Short sales are known to be difficult.