This is a tricky one.
In this situation, outing them on the forum is potentially one of the only trump cards you still hold. It might among be their only remaining motivation to pay you. Once you've outed them, you lose that. In addition, you might piss them off to the point where they use that as an excuse NOT to pay. "Well I was gonna pay you but then you went and aired our dirty laundry so now I'm not going to." They use it as a way to rationalize and justify it in their heads.
On the other hand, in the interest of protecting the forum population in general, you don't want to wait so long that they have a chance to stiff someone else.
I suppose it depends on the level of contact they are maintaining with you, if they're making partial payments, etc. In general I would say a month with no payments, a week of which there is no contact, warrants outing.