Whats the threshold for personal messages before you are banned for spam?
There are no fixed numbers, it is a case-by-case basis.
To perhaps elaborate on this, it depends on how/what the user is spamming, and partly how the user conducts themselves on the forum. For example, posting 3-4 large advertisements around the forum would (I hope) give you a temporary ban, however posting 3-4 low quality comments on certain threads would likely be fine.
I also believe that those part of a Signature Campaign are scrutinized more harshly than those who aren't, as they would perhaps be rewarded should they spam.
Would think exceptions would be made for people running a business here or is it frowned on to run a business here?
Why's that? Running a business doesn't make you exempt from rules in real life, why would they on this forum?