The only problem with importing now is if you're not going live with it. Any posts made on your live forum between now and the time you do go live would be lost and you'd have to reimport from scratch.
Importing all of the posts is the longest step, especially from UBB.classic because there is so much it has to do in order to get a clean import. I'm not sure on an average number of posts per hour as it depends on the server speed and how busy it is during the import. One of the guys that have done a classic import might be able to give a better idea.
