You can get an estimate of fatigue life off the maximum stress in the spring and a simple graph but it would be very heavily dependent on the heat treatment. Another difficult factor is what is the maximum stress in the spring? If the airgun is properly set up it would be a lot less than if the spring was buckling and the piston was smashing into the end of the cylinder. Springs have inertia (equivalent to adding 1/3 of their weight to the piston) and you can't stop that instantly. Too many variables to work out but I guess with a decent spring and a not over-stressed gun it should be tens or hundreds of thousands of cycles.