I had a client for whom I was getting reference requests on a steady bi-monthly basis for over a year after we saw each other (once). I finally cut him off and explained that it was rather a breach of etiquette.  

9 months? Yes. We providers generally have a preference for under a year. But QUANTITY of references is also a factor. I would say five is pushing it.

I recently gave a reference for someone who used the agency 2.5 years ago, and I made it clear to the provider that it had been that long.