How to Know that a Piece of Music is Mastered
1) The vision of the piece is mature and sophisticated.
2) Accomplishing the piece has gone into subconscious area and can just happen without active interruption. So the brain tension is quite low.
3) At the same time there is full awareness of what is going on while performing the piece. The possibility of watching your own playing process from aside occurs.
There is no limit of developing a piece. But the imagination of something can be mastered.