iternabe Or, how about by time? Like, learn 60 minutes of music per year?
Personally, I think it's fine for people to tweak the challenge to suit their interests, abilities and individual goals. So, in that regard, this would be fine. But if your goal is to learn a lot about music, to become better at learning new pieces, and to get better at reading (and read-playing) widely across piano literature, then I think tweaking it in this way misses the point.
Because the point, as I understand it, is that in order to learn a lot about music, to become better at learning new pieces, and to get better at reading (and read-playing) widely across piano literature .... to do all these and do them better, the recommendation is to learn a large number of discrete pieces, different pieces. Because it is that activity specifically, moving through a lot of music, a lot of individual pieces, that is the key.
So if you set the goal at 60 minutes, and then learned 4 15-minute pieces, this would not be aligned with that goal.