By now I have probably spent 1000 hours learning (as part of a formal process), reading, listening, writing and speaking English. Probably a lot more, but let's go with that estimate. The actual language learning part was probably 20%, the rest was acquiring cultural references.
I'm trying to do the same for French, where I'm probably around level 2 or 3, and damn, I had forgotten how long it takes to "load the cart".
In my opinion, native speakers actually "become" "native" in a lesser degree because they were born to parents speaking that language and in a higher degree because they went through school in that language. All the basic lessons at school that everybody learns, all those initial social interactions, day in, day out, in the language they will use for the rest of the life.
It's a lot harder to squeeze those in later on, especially since the interlocutors become less and less accommodating.