How long does it take for a brain tumor to get big?

Varies: Different tumors grow a dfferent rates however the start small like with a single abnormal cell and the cells reproduce at a rate faster than normal cells and no not die at the same rate as normal cells. This eventually leads to exponential or near exponential growth. In other words, they take a long time to get to a certain size but after they get there they grow more quickly.

Highly variable: There is no one kind of brain tumor. Some grow very quickly, others more slowly. So without knowing the age of the patient, the type of tumor, etc one can't reasonably answer this question. It may be years, it may be only months, and for a few rare rapidly growing tumors it can be just a few weeks or even days.