And how often and how long do they come before a brain aneurysm ruptures?

Unknown. Average age of rupture of brain aneurysms is 52. It likely takes a few years but this is so variable. Aneurysms are rarely seen in kids or people less than 35 years of age. Additional factors such as uncontrolled hypertension, smoking, connective tissue disease may accelerate the process to an aneurysn rupturing. Using Cocaine is also known to accelerate rupture.
Unknown. Brain aneurysms are usually diagnosed after they rupture. It is unknown how long the aneurysm had been present. If by accident one is identified prior to rupturing it is wise to treat it before it does rupture.