A few years ago I was making at least 20 posts per day on the regular, and they weren't spammy (AFAIK; I wasn't getting many, if any, deleted by the mods).  Everything depends on quality as far as I'm concerned, but I don't think there's ever been a concrete definition of "burst posting".  It's just one of those things where you know it when you see it.

Also, from what I can tell it's only ever been a problem for people in signature campaigns.  I don't recall ever hearing complaints about people posting too frequently if they weren't earning money from it--unless it was truly spam, i.e., off-topic nonsense or advertising links and such.  If campaign managers are cracking down on burst posting, I'm all for it though I can't say I've found any examples of it myself lately.