In one of the support forums I monitor, an agent asked this simple question: "How long should I try out a vendor for sending traffic to my site if I'm getting absolutely no traffic? It isn't a pay-per-click arrangement, it's a monthly fee."
Simple question, right? Of course, the answer on this isn't entirely simple. There are several things to consider when evaluating this sort of traffic generation arrangement. I'll give the standard consultant's answer: It depends.