It takes about an hour and a half to import ~3200 members and 55,000 posts from my classic forum to threads 7, it runs smoothly tho now after much beta testing, just set the variables on the first page and let it run. I'd say it would take maybe 10-11 hours on your site. I have a few mods (calendar and page views) that it imports, so an unmodified classic will probably import a little faster.
If I were you I'd tell the members to get some sunshine for a day, that we'd be back better than ever when they get back
