To me, the banns being, an announcement three Sundays at the service, could mean only two weeks (up to three, only if required just after the said service).
In Pride and Prejudice, if Wickham and Lydia can marry so quickly, it is because although the agreement between Mr Darcy and Mr Gardiner is not totally settled until the Monday, as Mr Darcy came on the Saturday to announce he had found the two, they lost no time in asking the St Clement vicar to publish the banns; so, having asked for it on the Saturday D, the first announcement came on Sunday D+1, the second on Sunday D+8, the third on Sunday D+15, and the marriage came on Monday D+16.