So I’m actually hitting up the tmi.twitch.tv/group/user/<channel>/chatters endpoint every minute to see who is in chat. If that endpoint returns an error, then I just assume everyone who was in chat is still in chat, and try again in a minute. So far that seems to be fine.
The API endpoints for subscriptions/followers, however, are paginated and to get the complete list might take a large number of concurrent requests. From this post here where a twitch staffer says:
We do not support polling the full subscriber list very often
I assumed that once every minute was too often. But I suppose “very often” is not well defined, and maybe requesting all X pages of a subscriber list every 60 seconds is just fine. But assuming it is too much, I realized that, since the list is ordered newest first, I can stop requesting additional pages once I see a date older than the previous request’s newest date. Then in the typical case I’m only making one or two requests every minute. But that only tells me when new subscriptions start. It doesn’t tell me when an old subscription has lapsed.