Mechanics

Conversions
1 Gallon = 6 Mugs

Rule
For every mug of ale drunk you get ADVANTAGE on CHA saving throws and DISADVANTAGE on DEX saving throws. Every mug of ale adds 2 hours to the duration. For every drink after the sixth roll a CON saving throw with a DC of (10 + every drink after the sixth). On a failure +1 level of exhaustion.

Example:

If a player drinks 8 mugs of ale then they roll against a DC of 11 (Base of 10 + the one drink after six) for the 7th mug of ale. They would then roll another CON saving throw against a DC of 12 (Base of 10 + the two drinks after six) for the 8th mug of ale. If a failure happens on either of the saves then the character has +1 level of exhaustion. If failure happens on both of the saves then the character has +2 level of exhaustion.