You can learn anything in your spare time--we have no idea how much spare time you have.
It also depends on how well you want to learn it. The simplest aspects could be picked up in several hours; you really *can* write a simple blog or whatever in a matter of an hour or two.
Beyond that? Totally depends. Do you already know Ruby? If not, most of RoR's magic will be completely undecipherable if something goes wrong and you have to diagnose anything. If so, it's a lot easier... but metaprogramming is very difficult for the unprepared.
subject: Is learning curve for Ruby on Rails short?