How quicky does dental pH change after eating?
Dental damage is mainly caused by the demineralization of the tooth enamel. Demineralization can happen due to a low pH caused either by acidic food/drinks like fruits or carbonated beverages or by bacteria producing acid while breaking down sugars.

Measurements (1, 2) have shown that food intake changes the pH (increasing or decreasing, depending on the food source), returning back to baseline after 15-30 min due to the buffering capacity of the saliva via bicarbonate and phosphate.

The drop in pH happens immediately after food intake. If teeth are not cleaned, the acid producing bacteria can have a longer-lasting effect on the pH for more than an hour. In these studies, participants are asked not to clean their teeth, so bacteria will populate the mouth. Removing bacteria or their food source as well helping the remineralization with fluoride toothpaste can counter the effects.