I also used to wonder about when I should move on and used to get so bogged down trying to learn to play the practice songs "perfectly" that I was making no progress. I was advised to focus on learning the concepts of the lesson and use the practice song as a way to learn the concepts, without completely mastering the song. The chord changes at full speed were the main problem part for me but I could get the basic moves after some practice.

Since my goal is to play various songs I'm familiar with, I'm simultaneously working my way through learning the concepts in those Fundamentals lessons with Lisa while learning a beginning-level song in the Songs section and for what it's worth, that's been a much better way for me personally to learn without getting stuck for weeks. But I've bookmarked those practice songs and periodically come back to them (and I tend to struggle less with them overall when I come back to them).