I've personally never had to replace a gas cap and they've lasted just as long as the car has.
A gas cap could go bad though and not seal properly so if the gas cap is not sealing correctly and causing your check engine light to come on then it would be time to replace the gas cap.
A gas cap should last 50,000 to 150,000 miles or more according to some manufacturers but usually gas caps last as long as the vehicle.
You can however replace your gas cap with a locking gas cap which would be a good idea to prevent gasoline theft from your vehicle.
If your check engine light comes on and there's no other issues and you've tightened your gas cap enough and the check engine light is still on then it could be your gas cap.
A new gas cap is pretty cheap as well though so it's worth changing it out and I would recommend if you're buying a new gas cap to buy a gas cap with a lock.