I won't consider quitting until I think my videos are the absolute best that they can be.
My growth is slow, but at the same time I feel that there is lots of room for improvement in my video quality.
So in the meantime I will continue to focus on researching, storytelling, audio, animations, pacing, etc..
That's basically it. I wouldn't quit unless I was putting out the 'best of the best' and still getting no results.