To my best recollection, I was on decadron for over 2 years. They tried to wean me down after a few months of treatment, but I had two more grand mal seizures despite anti-seizure medications. Twice, MRI scans showed minor increases in the tumor size, but my neuro-oncologist attributed the change to tumor neucrosis. Apparently the treatment was causing the cells to die more quickly than my body could absorb them, which caused the inflammation. After 1-1/2 years, the tumor finally began to shrink.
That said, aggressive tumors also produce necrotic tissue so brain inflammation should not be misconstrued as a sign that a treatment is working. Only a doctor can make that determination based on MRI scans and changes in the patient's clinical condition.
I'm not a doctor so I can't advise you on what dose you should administer or how long, or even if corticosteroid therapy is an appropriate treatment. If your mother is experiencing tremors, the doctor may want to prescribe anti-seizure medications. I urge you to continue to consult with a doctor about any course of treatment you take.