There are a variety of factors. Some primary tumors, such as a glioblastoma multiforme (GBM) have a life expectancy of less than a year sometimes only a few months. However, we have had patients that have survived with aggressive treatment, against the odds, for a number of years. Some other low grade tumors, such as an astrocytoma or or oligodendroglioma have a much longer life expectancy with treatment, well over five years. There are other non-malignant tumors that do not particularly affect life expectancy.
On the other hand if the cancer in the brain is a metastatic spread of the disease to the brain from a primary tumor, that can significantly decrease any life expectancy. In this case it is dependent on how far the disease has spread and whether it has infiltrated other organs or the skeletal system. Again, aggressive treatment can often prolong life for a number of years if the cancer responds and if it is caught early, if not, it can be a little as a few months.