I did an import this past week that took about 8 hours, it really depends on your forum size. If you have a nice server it's possible to bump up the number of topics pulled in at one time, but you'll have to take responsibility if something happens and you have to re-import. I've doubled the number on a generous server without issue tho.
Another way you can do this would be to close registration the day of so your member data won't get corrupted, warn people to not edit their profiles once import starts. Open a temp forum for them to post in and close the rest. Do the import and as it gets toward the end close the forum completely until it completes. A bit risky for data corruption, but if you keep tight watch on the process it's not too bad.
Myself - I tell my users to go get some fresh air, I'll be finished when they get back... after all, it's still just a forum - pay attention to the family for a few hours and everyone will benefit
