Each lesson-chapter is designed, I believe, to be mastered before moving to the next chapter. When I went through book 1 with my kids, each child had a different rate of completing. You have to your own judgement if your child is ready or not. My son was to book 2 very quickly, while the younger needed more time. They thought most of the material was fun.
I really like Professsor B, especially the beginning stages. We went onto Math U See a few months ago because my oldest was hitting upper level and I felt it was time to stick with a simple, solid program like MUS. I liked it so much that all my kids are doing it now, and have been very successful with it.
Not sure if that helps much. But just take it one page at a time and you should get the hang of it.