If you are interested in trepanations you should check several articles in the book "Trepanation, History, Discovery and Theory" (2003) edited by Robert Arnott. Specifically on the healing process, you should check its 3rd Chapter "The Pathology of Trepanation: Differential Diagnosis, Healing and Dry Bone Appearance in Modern Cases" by Nerlich et al.
Thank you, Claudia! You're always helping me! I know this article. There's no exact timing of regeneration of the skull bones. It requires my reviewer. Thank you dear Claudia!
I am no specialist in bone regeneration, but I think that (as with other healing processes) it will vary depending on a number of factors both intrinsic to the person submitted to the process (sex, age, nutrition, other pathological conditions, etc) and to the environment. I believe that considering prehistoric (or pre-industrial) populations in cold climates would heal much slower than others in milder climates.
Your question is interesting indeed. If you find good references, share, please!