One PC of the game I am currently hosting has the spell “aboleth’s lung”. In short it means, that any land creature cannot breathe air anymore, but only water. This lasts one hour. Of course she will use it as a weapon against some enemy, be it a dark elf, a minotaur or any other creature.
What are your suggestions to decide how long an enemy can still fight holding their breath? A minute (10 rounds)? A few seconds (1 round)? Are there any written rules for that?