To tell you the truth, I learned CAD while designing cases for the electronic products in my previous startup, and it took literally weeks; not more than a month. To get started, I went through several steps of the SolidWorks tutorial, then started over on things I wanted to make. The easiest package for me was SolidWorks, although the latest versions of AutoCAD are just as easy. While SketchUp is easy, it's awkward to to CAD-y things like draw a line and simultaneously specify its length (at least, that was the case when I tried it a year ago).

In less than a week you should be able to make useful stuff. These are the kinds of programs, though, that it is easy to forget much of what you learn after you don't use them for a while.

Rhino has an evaluation version which is limited to 25 saves. They have very good tutorials, extensive documentation and an amazingly helpful support team and user group.

It isn't that hard although the pro packages can be very intimidating. It's more a matter of time commitment than conceptual difficulty. Sketchup is pretty easy, I'd start there.