Since the first 1/3 of every math book reviews what was lost over the summer, if you keep at this program steadily and don't stop in the summer, you can make incredible progress. When ready for a new book, I have my student just take the unit tests that come at the end of every five lessons until we "find our place". Usually, if they have been doing math regularly, they can jump into the next level book skipping lessons 1-15 or more. This is great to know if you feel behind in math--you can catch up!
Tanya Khovanova's Fun Math Book Reviews