I could probably get mine off in about 10 minutes. With the shad rack, the tricky part is the installation. See here for my experience:
http://www.600riders.com/forum/fz6-general-discussion/19744-shad-side-topcase-install-many-pics.html
I think (but cannot guarantee based on personal experience) that if you remove one of the tack welded nuts from each side, installation and subsequent re-installation should be a lot easier.
When I first installed my rack, I thought I might be taking it on and off, but I found that I just leave it on all of the time. For around town riding, the SH29 top case is great for tossing things into. I installed a large sears top case for a 3 day trip in May, which also worked great. If you take the case mounting plate off, the rack itself is pretty unobtrusive, but I can see that you might prefer to take it off if you weren’t going to need a case for a while.