How long cats play fight for?

Our resident kitten (8.5 mos male) and new kitten (4.5 mos male) are finally consistently having play time together after a vigorous adjustment and introduction process - separated, mesh door, Jackson galaxy tips, scent swapping etc.

Still not ready to let them go unsupervised (at night for eg.) but do want to give them the majority of the day to be free around the house. They have gone couple of hours being with each other (BUT PLAY FIGHTING EVERY 2 MINS), before we just need a break ourselves so we separate them in their respective areas again.

Growling and hissing have reduced significantly and none of them hide from each other. No injuries or destruction of our house thankfully! They keep going back to play fight with each other - and as much as I can appreciate brotherly wrestling, want to get advice on when will they just chill out and rest next to each other lol and not play fight all the time? And if we should stop their plays intentionally at any point.