It took me 9 weeks to get my line cleared ready to place a new order when I moved ADSL ISP the first time (from Easynet). Thankfully I expected there to be big problems (though not 9 weeks), so I had a second phone installed and ordered ADSL on that before canceling the other ADSL service. Thankfully thinks are much better nowadays with cancellations or migrations...

...it can still be a painful process though.