I've never really been involved 100% in an upgrade project, we have a team dedicated for that in my company.  However, I think it's usually 4-6 weeks, depending on the scope.  The main thing to take into account is, #1 licensing, ensure you have all the proper licensing ASAP.  #2 hardware, for example, CVP has greatly improved their port density in later versions which allows you the opportunity to remove some servers from the solution.  #3 upgrade process, you can do a concurrent upgrade, where you're running two version at one point and can only upgrade one piece at a time, this process usually takes longer, but provides virtually no down time.  eLoyalty (my employer) has a pretty big presence in the Midwest in case you're interested.

david