If your radiator cap is the metal kind and has a rubber seal inside it then you should only need to replace the radiator cap when the rubber seal starts to crack.
I've had some radiator caps last 10 years and some I've seen lasted 20 years before they needed replacing.
The radiator cap itself should last a good while but it's the rubber seals inside that break down and crack which causes the radiator cap to not seal properly.
Some people replace radiator caps every 5 years or every coolant flush.
If it's the plastic kind of radiator cap then you usually do not need to ever replace it during the life of the car.
On my older trucks I replace the radiator caps every 5 to 10 years because they are not very expensive.