Once an offer is made to the Seller & the seller signs off on the contract (which should have a short sale addendum att'd), the 45 days could start there. The addendum indicates the lender has 45 days to approve the purchase price & contract terms. At the 45 day point, either party may within 5 days cancel the contract.
If the contract is approved by the lender within the 45 days, they usually look for a very quick closing, but the terms of the contract should prevail.
If you need additional info I can fax or e-mail more Buyer's info about short sale properties and a blank short sale addendum.
Have a great day.
Eileen R. Bishop
Realtor, GRI, ABR, e-PRO
Coldwell Banker Res. R.E.
erbishop@coldwellbanker.com