Czardha's page

So the Tetori monk can spend 2 ki to use their Grab on creatures larger than they are.

For the 1st round using Grab, that makes sense. You attack, you hit, you spend 2 ki to grab that giant.

However, Grab has an ongoing effect.

"If the creature does not constrict, each successful grapple check it makes during successive rounds automatically deals the damage indicated for the attack that established the hold. Otherwise, it deals constriction damage as well (the amount is given in the creature’s descriptive text)."

So because the Tetori has Grab, each grapple check it makes in successive rounds deals unarmed strike damage. This is in addition to Constrict, and in addition to the 'damage' grapple action.

My question is, how long does the Grab last? Is it 2 ki to establish the Grab, and then they have it till they lose it? Or is it 2 ki per round to keep having the Grab ability?

Actually, a creature can hold its breath for 2 rnds / point of Con, so that Mage w/ 10 Con can hold his or her breath for 10 rounds before it begins to suffocate.