What would be the average lifespan of someone with aortic stenosis?

Depends: Slight stenosis and minimal pressure gradient can be tolerated for a long time. Example congenital bicuspid valve patients can be aged. Once symptomatic aortic stenosis with syncope, angina, heart failure, the historic data suggest less than a year survival. That's why these patients are identified by murmur, echo and treated surgically.

Vague question: You need t know the gradient across the stenosis. High gradients require valve replacement, as the left ventricle will begin to enlarge (hypertrophy) which can eventually lead to heart failure. Echocardiography and/or left ventricular angiography performed by an interventional cardiologist can define the gradient, then you both can decide on best treatment options.