If my server goes down and someone posts from their instance onto my com does it still work for them then later federate to me?
Yeah. There's a retry queue, which does expire after a certain time period, but for a short outage that's how it'd work.
Do you know about how long it is?
I don't recall exactly, but it's more like days rather than hours. At some point the instances will mark you as down, and then stop trying to federate with you, so there's a hard limit but it's fairly generous and not especially aggressive.
I found the PR for the queue, and it mentions retries but doesn't seem to mention exact timing, at least to my quick read. ( https://github.com/LemmyNet/lemmy/pull/3605 )