At what age does the mouse skull stop growing?
This doesn't fully answer the question, but for those mouse skull sutures that do fuse, fusion is complete by about 45 days of age, according to Studies in cranial suture biology: IV. Temporal sequence of posterior frontal cranial suture fusion in the mouse.  That suggests that most cranial expansion is more or less complete by about 6 weeks of age.  I think there is still the potential for remodeling of the skull, especially bone thickness and so on, but since your interest is on the internal size of the cavity, this may help address it.