Does your home need a new cable installed?
If so, that work is contracted to a third party and the agent will not be able to give a firm date for that.
When I signed up on Monday last week, the date I was able to select on the sign-up page was changed to one three weeks later when the order confirmation was processed.
However, the cable pull and outside box were fitted on Friday and VM called me yesterday to bring forward the go live installation to this coming Tuesday, so if all goes to plan that'll be 8 days between ordering and getting up and running.
If you go to the uswitch website you can get to the "choose a date for an engineer page" without having to complete the full order so that may give you an indication of timescales in your area.