How long can a person live with a brain tumor untreated?

Highly variable: "brain tumor" covers many kinds of tumor, some fully benign and very slow-growing, and some highly malignant and rapidly growing. If one has a highly malignant brain tumor and is not treated at all, it may kill within a few weeks of discovery, or even sooner. For benign tumors, failure to treat may lead to serious disability or even death years after the diagnosis is initially made.