Dan;
The film is alkaline at about pH 9 in dmin regions and about pH 7 or less at dmax. The diffusion gradidient of hydrogen ion is driven by the pH gradient and through a gelatin lattice is rather rapid.
It depends on developer, development rate, and film.
Generally, diffusion of the whole content of a solution is complete within 15" which is normally considered the minimum time of full wetting of a dry emulsion. Hydrogen ion can move faster than that, usually taking on the order of 5 - 10 seconds in a wetted film or paper.
PE