How long do most people survive brain stem glioma, before it kills them?

Variable. 'brainstem glioma' as a term covers a variety of very different tumors. The classic pediatric 'brainstem glioma' is a diffuse infiltrating high grade tumor of the pons, and it is usually fatal in less than two years from diagnosis. Some children and some adults have focal brainstem tumors, more commonly of the midbrain or medulla, and these may be surgically removable and have a long survival.
Depends on... In adults, brain stem gliomas have relatively poor prognoses that correlate with histology (when biopsies are performed), location, and extent of tumor. The overall median survival time of patients in studies has been 44 to 74 weeks.