My mini poodle has had Immune mediated thrombocytopenia. How long can she be on prednisone safely?

It is a trade-off of living with prednisone and its effects or dying from an autoimmune disease that is eventually fatal. There really isn't much of a choice in the decision. I am uming your dog has an autoimmune case rather than one caused by a virus or infection that might clear up? If so, it is a lifelong battle to suppress her immune system so it doesn't attack her own body.