It's a question I faced and continue to face. Partly based on my experience with lessons decades ago, I didn't move on from the Fundamentals I and II lessons until I knew/played them 'very well.' My logic was that the lessons were putting down very basic skills and techniques...that the lessons would continue to build on what came before and it was therefore essential that I had the stuff 'down pat.' Later in Country Styles Level 1 the idea seemed to be validated when I came across admonitions to learn the lesson I was working on to the point I could play it perfectly...because of the importance and 'basics' of what what being taught.

While I was still in Fundamentals I gradually added to my practice time. Somethings I added were outside the Fundamentals course but most of the added practice time was devoted to things already covered in the GT course. In this way I could move on to new lessons but still work on that which still needed work or things which could easily justify daily practice no matter what the lesson, e.g., chord progressions, scales, versions of 12-bar blues, etc.

In the dim memories of my lessons when I was a teen, I can remember being frustrated by new things being introduced or attempts at progress that were stymied because I hadn't built up the supporting skills well enough. These days it's wiser for me to lean to the other extreme...get it down and get it down well before moving on. The idea of gradually adding to the practice sessions kinda helps me do both.