Hi,
St.Jude valve is a mechanical valve made out of metal and other synthetic materials consisting of two semi circular discs that open and close during systole and diastole respectively. This valve can last for up to 20 years if proper anticoagulation measures are taken, because there is a risk of blood clot formation.
It is important to consult your cardiologist to assess the durability of the valve that you have to decide upon replacement of the valve. Best.
I have a brother in law who had the same surgery as you, and they told him he would have to have the valve replaced in about 10 years. I'm don't remember why they said 10 years, though, just that he would have to have it done, sorry.