[Jadey]

I have a test Exchange BES, all one one box, around same number of users as you. SP2 install took around 20 mins. Took me longer to download it than install it hahahaha

Obv on a live production box you need to factor in some sort of disater recovery plan in case the upgrade spoons it, but in raw terms, just running the upgrade where everything is on same box, you're prob looking at around 20 mins.