Shouldn't take most that long but I had a lot of custom code to wade through. That and some of my pre-packaged mods were even customized further (one mod took me the better part of two days to get through). As it was my first time upgrading x-cart, I wanted to be reaaaaaaaaaaallllllllllllyyyyyyyyyy careful. That said, I could probably do the same work in 30-40 hours next time around.
Wish x-cart was as easy to upgrade as drupal & wordpress!
Blayne