You can pretty much Know C inside of ten years of using it professionally. I swear after twenty years of using C++ I still come across things I've never seen before, the rabbit hole very deep indeed.
Now with the new C++ there's a ton of new stuff to absorb.
There's never a hope of knowing 100%, but it is reasonable to be familiar enough to know 80% with ten years of experience and 95% with twenty. By thirty if you're the kind to take knowing your toolset seriously, 99% is possible. That last 1% is the "you can't be serious" aspect of the C standard.
Now with the new C++ there's a ton of new stuff to absorb.