A character who has no air to breathe can hold her breath for 2 rounds per point of Constitution. 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 in order to continue holding her breath. The check 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.
Although the medium he breathes has changed, a victim of aboleth's lung is going to suffer the same fate unless he reaches water.
An Aside: On the Cruel Nature of the Spell Aboleth's Lung
The spell aboleth's lung is a 2nd-level, slow save-or-die touch spell versus foes without dispel magic and who breathe. That's... actually a lot of folks, especially at low levels. That the creature dies faster if it tries to murder the caster is tasty gravy.
The problem with the spell is when it's used versus the PCs. It's entirely possible for PCs to face foes who can employ the spell when the PCs are among the folks vulnerable to it, making it just a slow, unpleasant death sentence. Such spells are unusually unfun, making players feel helpless while also killing their characters. Usually, it's more fun--as the DM or the player--to just cast scorching ray at a dude than to slowly suffocate him while making him do math.
There's an argument that adventurers should be carrying water when adventuring, giving them the means to save their air-drowning friend. But this assumes someone trained in the skill Spellcraft successfully identifies the spell when it was cast. (Pathfinder removed from the Spellcraft skill the ability of D&D 3.5's Spellcraft skill to identify a spell that’s already in place.) If the Spellcraft roll's failed, the DM just describes the spell's effects--suffocation. Uninformed PCs are then supposed to make a leap of cartoon logic that shoving their friend's head underwater will save him, which makes about as much sense as stopping bleeding by sticking a sword in the wound.
The spell, without DM's permission, is supposed to be exclusive to gillmen. Encourage your players to enter into a gentleman's agreement wherein this spell's just used by gillmen... then don't use gillmen. The spell aboleth's lung is a war crime.