Sand - 3:15 am on Jun 25, 2012 (gmt 0)
Three, six months, maybe longer.
I don't know. I've done many dozens of CMS migrations (it's something I specialize in). When you change a CMS, just about everything is destined to change, and most businesses take the opportunity to redesign everything from their taxonomy to their layout. I don't necessarily like it, but from a business perspective it makes total sense and I'm not going to stop it.
In my experience, if you do everything right, the worst case scenario is a 25% drop in traffic for about 3 months. However, most of the time it's more like a 10% drop for 4-6 weeks before things return to normal.