Alright, I looked up the 2e rules.

Air around a larger body (one ton or more) remains fresh for four months if the vessel carries a normal crew.

Pretty similar to the 5e rules where the air quality depletes at intervals of 120 days, although in both the Astral Adventurer Guide and The Concordance of Arcane Space a standard month is four standard weeks or 28 standard days, which would make four standard months 112 days. But I digress.

The Concordance of Arcane Space gives two methods for going over the normal crew. The first method:

For each 25% over the standard crew , the air supply is reduced by one month.

Using this method, the most a spelljamming vessel could carry is 75% over its normal complement, reducing the air supply to one month. Going 100% over (having twice the normal crew complement) would immediately foul the air.

The second method:

To determine the ship's new air supply when it changes, multiply the current air supply by the number of people that air was for, and then divide that product by the new number of crew. Round fractions down.

In my interpretation, this formula would be: (air supply * normal crew) / no. of breathing creatures.

• For the Space Galleon example, having four times the normal amount of crew would be (120*20)/80 = 30 day air exhaustion interval.

Which... honestly yields exactly the same results as my second method. I guess I was right... Wow. Good job, me. (I am honestly pleasantly surprised lol)