Hello Jeremy! How long it takes to learn the command line is a pretty open-ended question. I've been playing with Linux for several years, and I am still learning the command line. Well, I could be a slow learner, but I think you will find that Linux has a huge set of commands, not to mention writing scripts in BASH or an other shell. Whole college-level courses have been devoted to networking, administration, and security. Personally, I believe the best way to learn the command line is to just jump in and use it. I strongly suggest keeping a notebook or journal, and writing down what you find out about various commands that you use. You would be surprised at how often you will come across a situation where you say, "gee, I solved that problem once, now how did I do it?"
I also suggest that you have a good reference book next to your Linux box. The book I recommend is O'Reilly's "Linux In A Nutshell" 6th ed. by Siever, Figgins, Love, and Robbins. This is a true reference book, like a dictionary, where commands are grouped by task, and alphabetically and each command has an explanation, switches, and examples of usage. It weighs in at over 900 pages. It is Linux distribution independent. It also covers BASH, the common command line editors, sed, awk, and a whole lot more. Now, notice I said reference book - this book is not a tutorial on how to use Linux. It is much like an encyclopedia covering the command line and other tools.
I am talking pretty low tech here with notebooks and physical reference books, but it is pretty hard to be learning something at the command line and looking up something online, both at the same time. Having said that, I think you will also find that googling commands and other things you run into in using Linux to also be an excellent source of information.
It helps also to have some particular tasks in mind. For example, get an older box and load the minimal install without the GUI, or load the server version. Ubuntu has both. Try setting up a network so that your main box and the older box can "talk" to each other and share files back and forth. I have 4 computers in my little network: I have a Windows 7 desktop, an Ubuntu Desktop, a network "server" running headless and at the command line. It provides storage, web development, and printer services to all the other computers. And I have a really old box (~12-14 years old) with just 128mb of ram running headless and at the command line, that I use for Linux programming, particularly assembly language. I haven't had this much fun playing with computers since the DOS days! Linux really does bring the fun back into computing. I've learned more about Linux networking, programming, and command line stuff doing this than in any structured college course. Good Luck, and have a lot of fun!
~todd