Distributor Caps and Rotors last quite a long time and are usually only need replacing every 60,000 to 100,000 miles or sometimes more.
I would recommend changing out the distributor cap and rotor every other spark plug change which is what I do.
I replace the distributor cap and rotor on my truck around 100,000 to 160,000 miles and it's never been a problem and it does increase engine performance and improves fuel mileage.
Some people and vehicle manufacturers recommend changing distributor caps and rotors around 30,000 miles which is okay to do so but that's really too soon.