I think there are too many variables for how long PMR normally lasts. My experience matches up with info from the Arthritis Foundation:

"It typically develops by age 70 and is rarely seen in people younger than 50. PMR may last from one to five years; however, it varies from person to person. Approximately 15 percent of people with PMR develop a potentially dangerous condition called giant cell arteritis."
--- Polymyalgia Rheumatica - Arthritis Foundation: https://www.arthritis.org/diseases/polymyalgia-rheumatica.

My first occurrence of PMR came at age 64 and lasted 3 and half years until I was able to taper off. My second occurrence came at age 73 and lasted one and a half years. I credit the shorter visit the second time around with PMR with lifestyle changes I made, more exercise, eating healthier, eliminating most processed foods and sugar. I do still have my treats but not like the old days.