I'm not sure if their is a formula for determining this. The alkalinity of your water and the makeup of the grist are going to be important factors.
If you are going to attempt one, I would say check the pH every 10 minutes or so and see how much it changes.
Drinking on the keg: BPA, Brown Ale, Dry Mead, Wee Heavy aged on Oak, CAP
Drinking in the Bottle: