I'm not sure it's mastery that matters. I rather think it is confidence.
If you are able to use whatever you are using to achieve what you want with confidence, i.e. without being distracted by worries about the "how to", you're free to focus on the "what".
That's what changes tinkering with tools into using a medium.

The level of mastery needed depends entirely on what you want to do.
Think about, for instance, the many great photographers who have never even bothered trying their hands at, say, platinum printing.
Or, in other words, mastery is a reative thing. You may need 10,000 hours of practice to learn one trick. Maybe only 8 to master another. And if that other is the one you want to use...