No, they don’t define it because as you correctly said, it varies depending on how busy the server chat is, and the nature of the message.
A busy server you might be fine posting every 4 or 5 mins. The volume of chat means it gets pushed off pretty fast. The same message every 4 or 5 mins on a small server would end up being almost all the chat and the only thing going on…so you might get the people there annoyed.
The CS forum reps have tried to explain it - it tends to be related to just how sick of seeing it your server is. If they set a limit of 1 formatted repeat message ever X minutes people would just adjust to post just outside the limit - still annoying the heck out of the server. Volume of posts and chat activity really does matter in perceptions.
GMs have some leeway on that of course and can make mistakes. That is what appeals are for.