Your go live date is set by OpenReach. On the day it can take up until midnight.
Tackling the question I thought you actually asked (😛), Dr Google yields:-
Five to fourteen days, apparently. Expect Now to be at the longer end of this.
It matters if you have no broadband and are trying to get it; but if you are just switching providers, then the broadband from the losing provider will continue until the activation date for the new provider, as long as you have gone about this by going to the gaining provider and letting them liaise with the existing/losing provider.
At this point, the relevant timeframe is the one alluded to by @gavs82008