How long a person with advanced aortic stenosis has left to live if you don't fix it?

Depends. Asymptomatic pts even with severe as have a good prognosis. 1 percent per yr death rates. With symptoms rates much worse. 25 percent 1 yr and 50 percent 2 yr mortality. 50 percent of these deaths are sudden. When pts have angina , pts usually have their event around 5 yrs from onset of symtoms , 3 yrs for syncope (fainting) and 1-2 yrs heart failure.
Depends on symptoms. Patients with uncorrected aortic stenosis have a poor survival: half of the patients that develop angina (chest pain) will die within 5 years, half of the patients that develop syncope (passing out) will die within 3 years, and half of the patients that develop heart failure will die within 2 years. Aortic valve replacement can pretty much normalize the life span and improve symptoms markedly.