I'd heard it was 4.

I know that I've said this before...I'm in favor of flagging ads by people who break the rules (people who top post their bikes daily and more). The only ads that deserve the "re" comments are ones that misrepresent what they're selling or are dangerous...things like bent forks. The rules on Craigslist are 48 hours between posts, and that seems like a fair and reasonable guideline.