For us it's all about the impression. Somethings take a little longer.
Think about it this way. The three little numbers on the back of your credit card. When you first get your card, they are there, easy to read. But a month or two down the road, the ink has worn off. You continue to read those numbers by looking at the impression in the card.
When a child is actively working on a new skill the information is right there. But that initial learning gets pushed aside to move on to other things. There needs to be enough repetition so that an "impression" has been made in the child's memory. So that when the time comes they can recall what they need to move on to higher maths.
For my son this impression comes between 15-20 problems. Anything more than that is overkill, less makes recall iffy. Since everyone learns at different rates, you need to identify what your child's impression line is.
