You are right ... there isn't much research about how long one needs to be on a biologic for PMR. When biologics are used to treat other autoimmune disorders, people take them for as long as they continue to work. Biologics can stop working because the body can form anti-drug antibodies against the biologic.
https://www.verywellhealth.com/antibodies-to-biologics-4846279
I have been on Actemra for 4 years. My first injection was January 1st, 2019. I liked the symbolism of that date. It was a new approach because I took prednisone to treat PMR for more than 12 years.

I currently do a monthly infusion of Actemrs simply because I mentioned to my rheumatologist that an infusion worked better than an injection. I was switched to a monthly infusion to facilitate a surgery that was planned. The surgeon wanted me off Actemra for a month before surgery and a month after surgery. That seemed like it would be impossible while doing weekly Actemra injections.

My rheumatologist and I agreed that 2 months between infusions with surgery in the midde might be doable. The Actemra infusion worked so well on a synovial cyst in my lumbar spine, that it decreased in size and mostly disappeared. With the small size of the synovial cyst, surgery wasn't so urgent. Synovial cysts are usually caused by inflammation.

That 7 mg dose of prednisone is a tough dose to get past. That dose was where I had problems too. I would return to 10 mg and try again. I did the same thing many times with the same result. That dose is associated with how well the adrenals can resume cortisol production.

Actemra was started when I was on 10 mg of prednisone. I tapered by 1 mg per month until I reached 7 mg. I was surprised the pain didn't increase so I tried to taper by 1 mg per week. When I reached 3 mg of prednisone, I wasn't feeling well along with some aches and pains. My rheumatologist thought my symptoms were consistent with poor adrenal function more than PMR. My cortisol level was low so I was "ordered" to stay on 3 mg of prednisone until I could be evaluated by an endocrinologist.

The endocrinologist expected my cortisol level to be low. She expected my adrenal function would take some time to improve since I was on prednisone daily for 12 years. I needed to stay on 3 mg of prednisone for 6 months before my cortisol level showed some improvement.

When my cortisol level was "adequate" my endocrinologist thought I could stop prednisone. She said tapering from 3 mg to zero wasn't necessary but it was okay to taper to zero depending on how I felt. My endocrinologist didn't know what PMR would do. I was thinking Actemra was doing a good job of controlling PMR pain so I stopped prednisone after a fast taper from 3 mg to zero.

It took me a year to get off prednisone --- most of 2019. I did have cause to go back on prednisone a couple of times but I could taper off quickly again as long as I remained on Actemra.

I still do monthly infusions of Actemra. We will try going to an infusion every 5 weeks soon.