Clarification on seeded snaps on an ISO

How long does a channel such as stable/ubuntu-18.10 Need to exist before closing to seed snaps on an ISO? Or do they need to exist for all ISO builds? And they exist until you are no longer building that ISO version? I seem to remember reading that it just had to be opened once with a revision, they could be closed immediately. I had set it up a while ago, and I am updating my personal docs… Plus since there is no mention of these details, I would likely also add to