What is important is that you can measure how long it will take you to learn a piece. If the time it takes to get the piece up to scratch is immesurable then the piece is too difficult and it could take x units of time to get it up to scratch.
I think it takes many years before you can say you play a piece well, the piece has aged with you, you have played it countless times, you know it in an out, you can practically play it in the dark. I think it is easy to fake a piece, play it with full concerntration on expression, your fingers, very tiresome, it is like playing for examinations where you have to learn music in quick time, and it is prim and proper only through excessive focus.
Total freedom while you play, is that playing a piece well? Being able to play without thinking of your notes and mearly being an observer like everyone else. So what does it mean to really play a piece well? Just so that it sound beautiful, or just if your hands can do it all without effort, or both? What is beautiful, what is without effort for you?