How long do you have to wait to post a new topic for the same item (on the wanted forum)?

I believe it's once your topic has gotten to the 4th page in the forum

Either when your topic drops off page 3 (onto page 4) or when you have your original thread locked by staff (ping the minimods of that specific forum, or file a ticket).

Ok, thank you both!

I should add that some forums have different rules about that sort of thing; the FAQ stickies for each forum should explain when there are exceptions. :)