It depends,, This depends on many factors, like your health and the type of brain tumor you might have. If the tumor is a benign one like ependymoma or a meningioma, surgical resection can bring remission or even a full cure and a normal lifespan is possible. If the tumor is malignant like a glioblastoma multiforme or even benign but in the spinal cord, prognosis can be very poor with a survival of months.
Unpredictable. Depends on the pathology of the brain tumor. Location, size , cell type, etiology are all factors that decide. There are curable and treatable brain timors as well , which if diagnosed and treated the person can live a normal life.
Depends on the type. Especially before modern imaging was widely available, it was fairly common for folks with meningiomas to spend years or even decades as "mental patients", only to have the tumors finally found and removed with good results. By contrast, an untreated glioblastoma is generally fatal in weeks.