What usually helps when my 11-year-old 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. At this age, a clear weekly target usually works better than adding several new goals at once. The aim is steady confidence without hiding mistakes or rushing past the skill that needs work.