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).