Would you slow the learning plan down if a learner who can only study at weekends has improved enough that the old lesson length no longer seems to fit?

Lesson duration can be reconsidered when the learning task changes. Base the decision on what now needs to happen inside the session rather than keeping an old format simply because it is familiar. With a longer gap between lessons, revision at the beginning of each session becomes especially important. It is usually better to fix the smallest blocking skill first than to keep adding harder material.