If it is a straight forward switch over where there is no need for a BT Openreach Engineer then it normally takes two working weeks to switch over from date of order acknowledgment for customers who use the BT infrastructure which includes Sky Broadband.
NowTV usually despatch the Hub Router 5 working days before your scheduled planned date.
On the date of switch over, plug in the NowTV Hub Router and wait for the three green lights to show on the front of the Router then you should be good to go.
The switch over could happen anytime up until midnight on the scheduled planned date.
I believe NowTV will take care of the cancellation process for the Broadband & Telephone, where if you are a Sky Satellite TV customer you need to inform Sky yourself if you wish to leave their TV Sat service.