I've trained quite a few people to fly over the years so I've been asked this question quite a bit. It's really a tough one to quantify. Kind of like saying "what is soft". But, I have come up with a definition that I give most of my students, as well as others I see starting out, and I feel that it's a pretty good yardstick.
"When you quit thinking about having to do the basics. "
So, what exactly do I mean on that one?? Exactly what I say. Think about what happens when you are training. Just think about your ailerons being reversed. You move the stick one way and say to yourself, "oh crap, I meant to go the other way". So when you start moving the sticks without having to think which way is the right way to go.
Also, when you start recovering the plane from bad situations without having to think about it. Same thing.
These things usually happen without you even noticing it. Just one day it happens, you start flying without thinking. When this day comes differs for every pilot. Some hit in a few days to weeks, while others may take months or years.
There's some food for thought!!!
Ken