If you sat down and devoted most of your working hours to learning After Effects you could confidently learn After Effects in about 8 weeks.

You can learn Online or Offline. 

Yes, you can download Adobe After Effects trial  for free. Your official trial will last 7 days.

Adobe After Effects is a digital visual effects, motion graphics, and compositing application developed by Adobe Systems and used in the post-production process of film making, video games and television production. Among other things, After Effects can be used for keying, tracking, compositing, and animation.