Cat panting?

Now, I know cat panting is generally never a good sign in general; and usually common/normal after hard playing.

Picasso pants after playing hard. This I know is normal. But what got me more curious over concerned is how SOON she starts to pant. With Monet, he'd play hard for a good 5 minutes before he'd start panting, with Picasso, its about a minute. When I see her start to pant, I drastically slow down the play, or tuck her in the other room to play with Monet and give her time to breathe.

I'd bring her to my ear, and her heart beat sounds normal, even, steady rhythms and no congestion in the lungs (of course, a vet would hear more, but if I am not hearing anything drastically, I am not worried)

My question is.. what is the average playing time before they generally start to pant? Is it sooner for kittens? she's about 6 months, I think?
Since she only pants after playing for a short time, it's nothing really for concern? She plays HARD, running non-stop and will NOT slow down unless I put her in the other room or stop playing, lol.

Crumb is a panter. If it's too hot in the car when we get in, he'll pant and stick his face in the air conditioner. He pants after he plays too.

I say as long as the cat doesn't seem to be in respiratory distress, and isn't doing it with no provocation at all, it's likely not much to worry about.

Noodles pants pretty quickly when we play hard. She does have a slight heart murmur the vet said. I would ask about that at your next visit. I just try not to her her go too long or to let her get too stressed out. I'm really not sure how long she goes before panting though. She is overweight so that can't help.