Creep and fatigue are well understood, and there is no particular problem designing something with a creep life of tens of years, or a fatigue life of millions of cycles, if that is what is required. Some materials have "infinite" life, provided the stress never exceeds a critical value.
If the spring is properly designed for what it has to do, it will last as long as the rest of the gun mechanism. If it isn't, it might not. There's not much more one can say about it, without getting into the specifics of a particular design.