Any character can hold her breath for a number of rounds equal to twice her Constitution score. If a character takes a standard or full-round action, the remaining duration that the character can hold her breath is reduced by 1 round. After this period of time, the character must make a DC 10 Constitution check every round in order to continue holding her breath. Each round, the DC increases by 1.
Among the things Toff Ornelos doesn't know about Glorio Arkona is that he has 26 Constitution.
In Toff's defense, this is not at all a normal amount of Constitution for a person to have.
He can hold his breath for a touch over five minutes before he even starts to sweat. Or, if he's spending his actions evading the osyluth and trying to find or force a way out of his stone prison, two and a half minutes. (He could totally kill an osyluth, even without sneak attack, even underwater where his weapons deal half damage. It'd just take forever, and what little damage it deals him isn't worth the time.) Even after that he'll likely last a minute or two before failing the Fortitude save and drowning proper.
Stone has Hardness 8 and slashing weapons do half damage underwater. His enchanted knife can't chip it. He can't get out of this on his own.