Comment by J_Sherz

The most effective time to consume oral rehydration solution would be after drinking. Alcohol is a diuretic causing you to lose more water than normal through urine, and eventually also electrolytes (although at first, your body will make an effort to retain them). So if you drink the ORS whilst still in diuresis, it won't have any negative effects, but you're more likely to urinate away the contents. After drinking, but before sleeping is anecdotally the optimal way to use ORS products for the hangover use case.