I feel like an instance that hasn't had anything posted in a week in pretty much dead. Do you have any rules like that?

Seems a bit harsh considering single user instances or very small ones. I can be away for a week but not dead.

No, you have to consider that Lemmy and its communities are small, growing or just started fairly recently. Not much you can do but post on it yourself and hope for some interaction. Crossposting helps, too.

The beauty of federation is that even if you're on a "dead" instance as long as you're federated with bigger ones it doesn't matter

Sure but you still want them to be there for when the next wave of people come over.

I'm one of the only users on this instance besides it's admin, and sometimes neither of us use it for a week or so. Certainly not dead, but just temporarily abandoned. I'd say after a month of no posts or comments it is nearing dead, and 2 months it's actively dead.

ive been driving the concept of an 'onramp' instance that might not have a lot of local content, but allows users access to the rest of the fediverse.

but then, im not running a lemmy instance (mbin!) and a ton of my instance traffic is microblog (mastodon/universeodon) in addition to all the lemmy nonsense.

Give it more time; but yes, past a certain point, they're dead

Just make sure they're past the finaced majority definition of dead (aka non profit) and actually dead. (Usually 60 weeks or so)