20 years? Lots of Japanese people speak incorrectly until they start working. Japanese has a complex grammar that is highly contextual. In other words, you speak differently with your friends and with customers. If you never learn how to talk to customers, you'll use inadequate grammar when addressing them. But that's not the answer you're looking for. Probably a couple of years for basic day-to-day grammar.

I don't think there's a spefic time frame, because it depends on the person and there's a lot of grammar forms and ways to say things. I think once you learn enough to be competent, then you can figure out more advanced stuff because Japanese grammar seems to be very fundamental and builds on itself.