Most of the time the bank is the servicer (middleman) and not the owner/investor of the mortgage. That means additional time to locate/communicate back and forth with the investor. When the bank is the owner it usually goes faster.
Do you mean that the bank hasn't even accepted/rejected or counter-offered your contract ? I can take a while for them to wade through the paperwork involved but typically they are quicker if they are going to counter-offer for more money than they are at accepting an offer and approving the short sale.
Sometimes the listing agent has not provided the bank with a complete short sale packet (which is volumes of financial info from the seller). If they were missing information, or it was not updated, this can delay the process if the bank has to keep asking for more paperwork.
I suggest you have your agent (if you are using one) keep in contact with the sellers agent to see if the bank is requesting additional paperwork. If the sellers agent is upfront on what they know (or even don't know) about what's happening with the bank it makes it easier for all involved to bear with the slow progress.
Here's a blog I wrote on the short sale process to better help you understand what's happening. A short sale transaction is not for the impatient at all.
The short answer is 90 days to 9 months. There are so many possible ways to screw up a short sale it will really depend on the listing agent, home owner, short sale negotiator (may be the listing agent or separate party) the banks process, the banks negotiator and the actual investor. Wells Fargo may service the loan, but Fannie Mae, Freddie Mac or any number of other investors may be the actual decision makers.
Wells Fargo short sales for me in the past were 3-4 months from list to close, and I have even closed in as little as 2 months, but,,,,again everyone is different.