How many minutes before it become visible that post was edited:
5 minutes.
For example, if 2 posts are posted back to back in very short time and someone edit first post using content of second post(blatant copy/paste) how will anyone prove first post was plagiarism and second post is good?
I don't think you can prove it. IIRC, theymos (or an admin/mod) said that the "edit history" only shows up after the 5 minutes, so they can't see what was the post if the user edits it before that time.
Source:
Edits aren't recorded when they occur within 5 minutes of the post time. (This has always been the case.)