Where does your retention/deletion criteria come from?

I would argue it comes from a bigger framing, a goal that you already determined. Or at least it should be the case.

Training a skill is part of learning. But it is not the whole picture. That is why public schooling fails. It does train you to acquire skills, just not those *you* decided to learn for your own reasons. Thus it fails to actually teach anything.

Clear goals first, practice second. Helps you stay motivated.