Technically, stamp duty is payable 30 days ( in or around) from the date of the deed - the closing date. Technically you do not have to pay it until then. Practically, your solicitor will insist that you pay it on or before completion - they are not supposed to release your loan cheque until they are in funds to stamp and register the Deed and your lenders mortgage.
If there is no mortgage, you could decide to take the extra time - my own view is that I want the deal closed and the Deed stamped and registered as soon as possible so that I can close the file and get on with another.
mf