Grade 3 at 10 then stopped. At 66 restarted last Sept - Moonlight Sonata 1st Movement took 2 1/2 months Pathetique 2nd Mmt 4 1/2 months - then Cool (ABRSM Grade 5) 1 month - then Bach Invention No 8 so far 1 month and is fluent but error prone - expect another 2 to 3 weeks to finalise. These would be performance ready, although I don't perform (other than to my teacher).
But since Mid November I have also been using Piano Marvel and have learnt over 90 pieces to a point where I score 100% in their assessment - working through their levels 1 to 5. levels 1 and 2 were trivial and I would knock many off in a day. Level 3 and 4 got harder and I would average a piece a day, Level 5 about a week a piece, Level 6 needs the level of commitment that my main pieces need and so my progress has pretty well stopped (currently on Bach Prelude in C - but it is required that I get it to an insane speed and score 100% to progress)
tl;dr It depends on difficulty and level of polish