Ask HN: How long does it take you to familiarize yourself with a new codebase?

Sometimes, I try to contribute to open source projects, but codebase of some projects are huge (chromium, spidermonkey, anything written using LLVM (Swift, Nim), ...)

How long does it take to you, surf codebase, find something for yourself and try to create patch for it?

I am mostly interested in deeper knowledge of C/C++ in order to improve my skills in high-performance, distributed computation area.

But whenever I see huge codebase and until I surf the code, find something for myself, another guy would create a patch for it, or issues look so hard to begin with, or not possible for me at this time (consider beginner issues which require special environment with some special device)

So how do you contribute? What would you do in my situation?