I didn’t realize your dad was already a patient at Mayo. From personal experience, I know the specialists in the hematology department do their utmost to find successful treatments for their patients.
It sounds like your dad was in remission for a while and now the plasma cell leukemia is active again. Unfortunately, even with medications, there can come a time with blood cancers like your dad’s where treatments are no longer effective.
When that happens doctors are reluctant to give time estimates for longevity because it is difficult to predict with a rare disease.
As the cancerous blood cells continue to proliferate in the bone marrow and blood stream, eventually they will crowd out healthy blood cells. This disease is related to multiple myeloma which can cause bone pain, thinning bones and broken bones. With the relapse of his disease there simply may no longer be any options available for treatment. With subsequent blood work, where his doctor can compare one month to the next, they may be able to track rate of progression and give a more definitive answer.

I don’t blame your dad for not being eager to try another round of chemo. Have you spoken with his doctor’s NP or Nurse Coordinator about palliative care for your dad. This is not hospice. I found a good article about palliative care for MM patients. https://www.webmd.com/cancer/multiple-myeloma/palliative-care-multiple-myeloma

How is your dad feeling? Is he fatigued or experiencing any pain or discomfort?