It all depends. I've seen people get up and running within a year, others in multiple. I've been using Maya for 9+ years now, and I'm still learning things from day to day. Don't get fixated on the time. Fixate on what you enjoy, and the time will be the least of your concerns. If you're looking to get into something that's quick, 3d is not it.
Best of luck.
Oh I have no need for it go fast by any means. I just like to have a general idea of how long it'd take me to get familiar with it, since it feels like looking up at the pyramids.. an impossible task