Yeah, I'm with Tolak, and 2002-William. If anything, I'd advocate that posts shouldn't even be editable after a short amount of time -- 1 hour seems right to me. There have been many cases where someone posts something truly vile hoping the target person(s) will see it, edits it to clean it up, and then feigns innocence. I hate to say it, but this tactic has been directed at me. I quoted the message to make sure it stayed, but that's not foolproof either since you can edit quotes.
I'm on a forum where after 6 hours, a post cannot be edited. I really like it, and as above, I think 1 hour is even enough time to correct spelling and grammar. If you think of something more to say later, add another post.