How long can you live a brain tumor and not know about it? Can you live 12 yrs with it and not know it?

Sometimes. Brain tumors come in different types, some benign and slow-growing, some malignant and often fast-growing. Clearly some patients go many years, even more than 12, without knowing they have a tumor, which can be shown to have been present if they happened to have had a head ct or MRI after, for example, an accident.
Depends. A very common benign brain tumor, a meningioma, may be present for years, remain small and cause no symptoms. Many times, meningiomas show up on mri's taken for a different reason. A more aggressive tumor, such as a glioma, could cause substantial symptoms within weeks. Neurosurgeons can guide indications for surgery.