under normal circumstances, posts, comments, and, yes, even edits should propagate through federated instances within seconds, minutes at the most.

however…

due to several factors, chief among which are version skew, network lag, server load, software bugs, and a couple of other problems, data propagation has not always been so smooth or working as expected. sometimes there’s a backlog of federation data or some data needs to be migrated during an upgrade… or something else…

it depends on the post/comment and the instance it’s on and where you’re viewing it from, and a lot of factors that are getting worked out. long story short: it should work, sometimes it doesn’t, but it’s getting better.

¯\_(ツ)_/¯