Use your judgement. You are the one who knows your cats behave.
If it sounds like one of them is getting hurt and is unable to get away, then that is the time to intervene.
With my two, I know that they hiss as part of their play. But there is one paricular hiss that I know means, "Get out of my face NOW." That's when I break things up between the two.
I also know that growling is not part of normal play between my two. And I intervene then as well.
There are also times when the Alpha position is being fought out. So, I let things be. I do not interfere unless one of them is going to end up hurt. I simply add a drop of Bach's Rescue Remedy to their drinking water during this time and spray some Feliway onto the kitty areas including their posts in an effort to ease the tension.
Signs Indicating that Things are No Longer Play
1) Aggressive hissing and growling.
2) Loud aggressive meows.
3) Meows of pain.
4) Meows that sound closer to howling.
5) Anything that you know that either one of your kitty does when they are angry, upset or very annoyed.
Things to do to break two kitties up.
1) Clap your hands loudly.
Say No sternly.
2) Clap your hands loudly.
Throw a couple of toys past them to divert attention away from each other.
3) If clapping or any loud noise fails to work, physically separate them using care as to not to cause injury to the cats or yourself. But in most cases, your approach towards them can distract them at which point you can divert their attention to a toy.
4) Use water. This is the last resort
. This is only when you have two kitties trying their hardest to inflict the most injuries to each other and nothing you will do can stop it. Generally by this stage, your two cats will literally be rolling around the place like tumbleweed. You don't need a lot. One quarter of an average sized bucket will do.
And if they're kittens, well, they do grow out of it.
It's their normal play and fine tuning of survival skills.