This is the main issue I have with the oft-quoted 45 min to 2 hours time span. You don't have to be an expert to know that the kind of head wound JonBenet received would have caused a lot of bleeding - and if the skin was intact, a lot of swelling. In fact, the two opinions we have directly from the sources (Wecht and Dobersen) both say the headbash was "perimortem", at or close to death, and cite the miniscule amount of blood and swelling as the reason.
The time span, which both Beckner and Kolar cited, appears to originate with Dr Rorke, whose opinion we only have second hand. The description Kolar attributes to her, with a brain swelling so massively that it comes out of the skull, is indeed a good description of what would happen from a headbash like this - the problem is that this is not what the autopsy describes. Either Beckner and Kolar (who I suspect got this from Beckner) misunderstood what Rorke told them, or (as I suspect) the whole thing was complicated by the garrotte, which was applied simultaneously. Either way I would love to hear from Rorke directly.