You'll become proficient in using Aikido for self-defense purposes in... about 10 years.
*tenkans* Oops, your punch missed. Excuse me while I dodge some more.
(No, I'm not so green as to believe that dodges will get you through everything...just making a point.)
Louis, I believe what's been said should suffice...but let me add that while that was probably the biggest motivation of mine, it wasn't the only one. Others included learning (anything...learning=good), satisfying a curosity I had had for several years, athletic activity, and learning.