It is definitely a catch-22. If you put an unrealistic time frame, a good agent will encourage their seller to pass on the offer. There is a lot of time and effort and expense incurred when submitting a short sale and nobody wants to go through all that if the buyer is not serious or not willing to give a reasonable amount of time to get it done.
On the other hand, and as others have already stated, it puts the buyer in a difficult situation if they don't. However, at least in the State of Wisconsin, the buyer can always ask for a release of the contract at any time. The seller doesn't have to agree, but it can be done.
As a buyer's agent, I always mark the addendum to reflect that all dates and deadlines run from lender acceptance. That way the buyer is not putting out money for earnest $, inspections et cetera until they know that the bank has accepted their offer.
The shortest and clearest thing I can tell you is, if you are in love with the house, don't give the bank a time line. Just know going into it that it could take awhile and MAKE SURE that the agent for the seller knows how to correctly submit your offer. This is probably the most crucial aspect of buying a short sale.
Wishing you good fortune in finding your perfect home.