how much practice, and for how long?

Thinking about the girl who learned 1 and a half year's math over the summer, I looked up Willingham's article, Practice Makes Perfect But Only If Your Practice Beyond the Point of Perfection.

If material is studied for three or four years, however, the learning may be retained for as long as 50 years after the last practice (Bahrick, 1984; Bahrick & Hall, 1991). There is some forgetting over the first five years, but after that, forgetting stops and the remainder will not be forgotten even if it is not practiced again. Researchers have examined a large number of variables that potentially could account for why research subjects forgot or failed to forget material, and they concluded that the key variable in very long-term memory was practice.