General etiquette is to kick it out when there's a clearly serious injury if the referee doesn't stop play on their own (they should).

But there's so much embellishment/faking serious injury now that you see it less and less.

I'm seeing more and more players need attention and remain on the ground for multiple minutes (and not something caused by a challenge). Yet, they get waved back on 5 seconds after play is restarted. I think that is BS. It's one thing if it's the result of a bad challenge, but I've seen it just from cramps.

We pull them right back on when play restarts.

The only requirement is that they leave the field (if play was stopped for them, unless they are a keeper).

I've seen players waved right back on and I've seen refs make them wait. If it isn't in the laws, I don't see what the 4th official doesn't just send them back on when ready.

The point is to allow someone who needs it to receive medical attention, not to allow a substitute. And it depends on the match situation. If it’s late in the game and one team is on attack, they needn’t feel obligated to stop for an injury that is likely not serious and is probably just an attempt by an opposing player to break up the flow of the attack.

Honestly, too much is made out of kicking it out vs. continuing play. The referee can stop play at any time if it's serious.