I learned Perl in my free time for low-priority hobby projects. After half a year or a year I felt comfortable. However looking back, I frown at the code I wrote back then, and call my earlier self over-confident. And I've certainly learned quite a bit about Perl since then.
Now it's about 9 or 10 years since I started to learn Perl, and I still learn new things about it.
It really, really depends on how much effort and time you put into it, and how much background knowledge you have (in how operating systems work, background in other programming languages etc.).