I think the 5.9 years as the "average" duration of PMR came from the following study.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6475501/
Someone on another forum repeatedly said the average duration of PMR was 5.9 years. Other people started to repeat 5.9 years so that it sounded like an echo chamber.

You simply can't take one small study and say the results are the facts of the matter. People who do that have some kind of an agenda.

Some people take prednisone for a short period of time and some people take prednisone for a long time. Some people take prednisone for the rest of their lives ... some people taper off quickly.

Some people have many side effects ...some people don't.

This study doesn't cite the average, it says the median. It also has a 95% confidence interval around the 5.95 number which means that based on limited information the median may be anywhere between 3.37 and 8.88 years but only at the 95% level of confidence.

In an interview, Dr Matteson said this about the study. He was one of the authors of the study from Mayo.

"While the findings are somewhat good news regarding side effects, steroids are often not popular among patients, Dr. Matteson noted, due to other concerns, such as the weight gain associated with steroid use. The researchers did not examine weight gain. Nor did they look at appearance changes due to the drug or mood changes, also problematic for patients.

Dr. Matteson emphasized that these findings do not mean that experts should not continue to look for a better treatment options."