How soon following a lumbar fusion do guidelines tell you that you may begin interventional pain therapy. My last surgery was a salvage procedure.?

Depends. I usually advise my patients with this issue to check with their surgeon. In cases where there has been persistent pain, I have done spinal procedures as early as one month after a spine fusion. I don't believe there are any hard and fast rules to this.
There is no answer. The most important thing for you to do is find out whether you are in fact fused. If so, there are no limitations to any treatment. Pain management can be implemented at any stage after a fusion operation. Remember that all you will get from pain management is temporary relief. True recovery comes when you achieve independence from all therapists and medications.
Variable. There are no clearcut guideline when one can begin interventional pain therapy. Healing of a lumbar fusion begins immediately and is complete in 1-2 years. However, you can often tell whether you are getting some good relief within a few weeks. If your pain is not improving you may want to consider some further (typically non-operative) intervention.