A character who has no air to breathe can hold her breath for 2 rounds per point of Constitution. After this period of time, the character must make a DC 10 Constitution check in order to continue holding her breath. The save must be repeated each round, with the DC increasing by +1 for each previous success.
When the character fails one of these Constitution checks, she begins to suffocate. In the first round, she falls unconscious (0 hit points). In the following round, she drops to -1 hit points and is dying. In the third round, she suffocates.
Slow Suffocation: A Medium character can breathe easily for 6 hours in a sealed chamber measuring 10 feet on a side. After that time, she takes 1d6 points of nonlethal damage every 15 minutes. Each additional Medium character or significant fire source (a torch, for example) proportionally reduces the time the air will last. When a character falls unconscious from this nonlethal damage, she drops to -1 hit points and is dying. In the next round, she suffocates.
For example, two people can last for 3 hours, after which they each take 1d6 points of nonlethal damage per 15 minutes. If they have a torch (equivalent to another Medium character in terms of the air it uses), the air runs out in only 2 hours.
Small characters consume half as much air as Medium characters. A larger volume of air, of course, lasts for a longer time. So, for instance, if two humans and a gnome are in a sealed chamber measuring 20 feet by 20 feet by 10 feet, and they have a torch, the air will last almost 7 hours (6 hours/3.5 people and torches 4 10-ft. cubes = 6.86 hours).