I'm curious to see how far this routine will take me. Will it continue to work with 20 pieces? 25? Or am I already at the limit for the number of hours I put in?
Here is my four-month update.
I've managed to bring up three more pieces. So I have 16 that I would feel comfortable playing from memory in public. As expected, the practice gaps for most pieces have gotten longer. I may go a week or even two without playing Puerto or Sonetto 104, etc. But they don't seem to be suffering much. When I come back to them, the memory is fine and there's even a freshness to the interpretation that I'm really enjoying. I'd probably want to work them hard for a week if I had a serious performance, but I'd have no qualms about whipping them out for something more casual.
As for new pieces... I'm working on two big ones and three little (tiny) ones. The big ones are a Rachmaninoff prelude and a Mozart concerto. I played the Mozart a hundred years ago, so the learning curve isn't too steep. It's longer than the solo pieces, so that takes time. I've never played the Rachmaninoff before (or any Rachmaninoff). It's a challenging piece and requires daily attention. But I'm definitely over the hump and it's getting close (mostly memorized, almost to speed).
The three tiny pieces are from Schumann's Scenes from Childhood. Mom sent me the music and I chose three of the more popular ones to learn. It's good to have some short, popular tunes in the bag.