How long does it take for a brain tumor to grow large?

Depends: For more benign tumors such as meningioma, many years. For malignant brain tumors such as glioblastoma, a few months.

Brain tumor: This is an extremely complex question. Brain tumors come in many different types and based on the cell origin can be extremely slow growing taking years to become large and symptomatic versus the more malignant which can double in size in a matter of months.