How much symfony do I need to learn if I am going to be a CiviCRM core hacker?
You must be digging pretty deep if you're running into Symfony. Its introduction into the stack is fairly new. CiviCRM has been around for over ten years, so some of the technologies are pretty hip and new (like Symfony) and some of the technologies, like Smarty or PEAR DB... well, not so much.

I think I've run into Symfony in some code around events (like... programming events, not CiviEvents) and maybe in some of the new command line tools. If your primary concern is fixing bugs, you may never even see it, as many of the bugs tend to live in older code.

Bottom line: It's a technology the core team would like to adopt, but adoption is happening slowly. I wouldn't say that Symfony experience is a prerequisite for developing CiviCRM any more than AngularJS experience is.