I would say I have been studying it for about a year (maybe 5 hours per week average, with some all-nighters), and still consider myself a novice, *but* have learned a lot. A lot!! I can change my templates, install plugins, and can really customize my WordPress. Many of the things I learned I don't understand really, but I got code from this forum or the codex, figured out where to put it in my WordPress, and it works.
After I decided WordPress was my best choice for blogging (this took me months of comparing), then it took me only a few hours to have a basic blog up and running (5 minute install, then my first basic customizations).
*IF* I knew or was learning CSS, which I haven't, then I think I would be able to do a lot more with my blogs designs.
The best approach might be, install WordPress, start blogging daily, and learn specific customizations as you decide what you want. Don't learn everything beforehand, just ask/study each new feature that you want as you think of it.
Look at many other blogs and when you spot a feature you want, learn how it's done.