an adult returning after a long break has improved enough that the old lesson length no longer seems to fit. What would a teacher normally work on next?

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. A returning learner should be checked for what is still secure before old material is automatically retaught. If the same difficulty keeps returning, show the teacher exactly where it appears so the next lesson can target it.