It is possible to make a post under 5 minutes which may not violate any forum rules so its subjective to the intention of the post not the numbers, usually campaigns consider there should be atleast 30 mins gap between their post or else it will be considered as burst posting then people wrote x number of posts in a notepad or somewhere then keep posting it in between the right intervals that is why most of the signature campaigns have max cap post towards post quota for weekly payments but you can make as much as you want.