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.
Here is limits on posting. Anyone who has an if (activity >= 100) then they can post every 4 seconds.
Limits on postingwaittime = 360;
if(activity >= 15)
waittime = (int)(90 - activity);
if(activity >= 60)
waittime=(int)(34.7586 - (0.0793103 * activity));
if(activity >= 100)
waittime = max((int)(14-(activity/50)), 4);
About spam, low quality posts, blown up posts, or off-topic it is something else. This means nothing is going to stop you from posting every 4 seconds assuming you have >=100 activity points unless you are spammer. But the time span between one post and another also needs to be considered because it can provide space for you to write something constructive and of high quality. If you are an AI, you may be able to have short timeframe due to the quality of your posts.