There’s actually nothing that flat out states this in the OD&D rules, but there are some hints.
A character swallowed by a purple worm will die in 6 turns. (Vol. II, Monsters & Treasure, p. 15)
The Bowl of Watery Death will drown a character in 2-5 turns. (Supplement I, Greyhawk, p. 52)The average person in the real world can hold their breath for 2 or 3 minutes, so if it’s assumed the above is in “combat” turns, it’s a generous but still close to accurate number. As usual, it seems “just roll a d6” is a good default ruling. Throw in the Con bonus for extra variety and start making saves or adversity rolls every minute when the time is up.
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