Front Brake Rotors

In replacing a leaky caliper My mechanic said the Rotor(s) need replacing. What is the opinion on how long Front Brake Rotors can last ? These have 15-20k on them, never been turned and are about 3 years exposed to snow, salt slush and water. With average wear and tear do they need replaced??

The insides of the rotors usually get pretty rotted. If they are than there not worth turning. A lot of material will have to be removed to get the inside surface clean again possible leaving the rotor to thin.