If you mean 'how long does it take to effect the changeover' - about 30 minutes. If you mean 'how long does it take from ordering to completion of provision' - how long is a piece of string.
if you mean how long does it take to have the upgrade working - mine was a month from ordering to the upgrade activation date.
and then a week without any internet at all as it didnt work ...
everyone will give you a different length of time from ordering to activation - some will be what you want to hear but some, from reading the forum posts, will give you nightmares. who knows which category your upgrade will fall in.
my own was about 10 days from ordering to activation and went without a problem