Suffocation Limit

A creature can hold its breath for a limited number of minutes before it begins suffocating, this is called their suffocation limit. A character's suffocation limit is equal to 1 + its Constitution modifier (minimum of 30 seconds).

When a creature runs out of breath or is choking, it immediately drops to 0 hit points, begins dying, and can't regain hit points or be stabilized until it can breathe again.