You have asked an interesting question. Nobody really knows how long PMR lasts except that prolonged treatment with Prednisone is usually needed. Some people say that for PMR the "average duration of treatment with prednisone" is 5.9 years. Saying the treatment lasts that long doesn't mean PMR lasts that long. The unfortunate side effect of Prednisone is adrenal suppression which occurs within 2-4 weeks.

I had another autoimmune disorder called reactive arthritis. I managed those flares with a high dose of prednisone followed by a fast taper. It was not uncommon for me to take 60 mg of prednisone and taper off within 4 weeks. I was treated by an ophthalmologist who prescribed the prednisone to treat the flare of uveitis that accompanied my reactive arthritis flares. I could achieve remission within a week or two and I was off prednisone again within 4 weeks. Another flare would usually recur within a year but I would be off Prednisone for 9 months and sometimes longer. The flares of reactive arthritis were very painful but the treatment with prednisone wasn't very long like it is for PMR.

When PMR was diagnosed, I initially tried to taper off prednisone in 4 weeks. A fast taper didn't work and that was when my rheumatologist said I would need prednisone for an "extended period of time" which turned out to be 12+ years.

I found the following on the internet:

"In essence, the development of adrenal insufficiency due to glucocorticoid therapy for PMR can create a cycle where the need for treatment is extended because the adrenal insufficiency symptoms mimic the PMR condition, making it challenging to safely reduce or discontinue the steroid medication."

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When my chronic PMR was treated with Actemra, remission was achieved in a couple of months. However, I needed to stay on low dose prednisone for 6+ months to let my cortisol level improve.