for me, if someone falls asleep or otherwise goes idle in a chat, I'd wait a bit then probably do smth else... if replies happen every 10 or so seconds, I'd wait 30s, then switch tabs... sound pings will bring me back if they return within a few mins to an hour or so... or I'd close the tab if I'm done with the chat.
It's slightly annoying when a tab is left open and a person shows up as online for hours, even tho they haven't replied to dozens of messages that the rest of chat is replying to... are they asleep, did they leave their tab open but autoplay failed, did they walk away from the PC, etc? one can only wonder, and maybe ask them later (all of the above has happened to me, tho the other ppl apologise, and it's more or less occasional... but autoplay being unreliable doesn't help)