How long can a person live with a brain tumor before it causes symptoms?

Depends. It would depend on location of the tumor, type of tumor and grade of tumor. A high grade tumor next to the motor cortex will cause symptoms quickly, a low grade tumor in the frontal cortex may take years.
It depends. Like any other cancer, it depends on location and other factors. A brain tumor in a vital structure will cause symptoms very quickly, but if it is located in a less critical structure, it may get quite large before any obvious symptoms.