A radiator cap should be replaced after 100,000 to 150,000 miles or if it goes bad before then.
Radiator caps are pretty cheap and are essential to the proper operation of your radiator to keep the coolant pressure at the right pressure.
If the radiator cap is not sealing properly and not allowing the engine coolant to build up enough pressure then the engine may not cool down properly which results in overheating of the engine and coolant boiling.
I replace my radiator cap every 10 years which is enough for me but sometimes I've had a radiator cap fail within 5 years but they are pretty cheap and easy to replace.
You just remove the old radiator cap and then put the new radiator cap onto the radiator.