When I am learning a new piece, I only work on that piece until it's completely and that I can play it perfectly all the way through. I don't even try to learn any other new pieces. I will still try to keep up with pieces I already know how to play though. Like I will play some pieces that I already can play from memory before I begin learning the new piece for the day or at the end. Anyway, for me learning a new piece all the way through correctly is a pretty long process. Possibly months and months, depending on the difficulty level of the piece I am trying to learn. Hope this helps some!