OH, if it is about a glass ceiling then... if such a thing exists, wouldn't it just be the point at which it is not possible to more accurately and efficiently express what you want to express using your art?
It's like, if you shoot an arrow and it misses the target, then it's easy to shoot better the next time. If you just shot close to the bull's eye, it is harder to improve on that. If you already shot a bull's eye, that's when it's impossible to improve. But in that case there's nothing to worry about since you already achieved the best possible outcome.
To me the bull's eye is when I can express exactly what I want to very quickly and efficiently. If I reach that point, I won't worry about not improving anymore since I'll already have what I want. After all, improving just for improvement's sake doesn't have a point.
I also wouldn't worry about reaching the 'glass ceiling' before reaching the bull's eye. If you haven't hit the bull's eye it means by definition there is more room for improvement, although you might have to refine your methods of practice to get there.