How quickly do bran tumors develope serious symptoms?

Depends on location: Serious symptoms from a brain tumor depend upon the location in the brain and the rate of tumor growth. There are parts of the brain where a tumor can grow to a large size with few symptoms and others where a small tumor can cause profound symptoms.

Depends: on the type of tumor, the rate of growth and the location in the brain. A small tumor in the right location can give severe symptoms and a large tumor in a different location may have no symptoms or minimal symptoms.