This is a superset
Basically there's no time limit. I forgot to mention that (of course) only the author or a moderator can edit, and the identity of the person making the edit is recorded too.
If you're saying that you prefer the 30 minute rule because there's no record of edits, well, that sort of thing goes against my grain. Even if I restricted changes to 30 minutes after posting, I'd record the fact that an edit was made and preserve previous versions.