You really can't put a milage factor on how long a rotor should last. There are so many variables other than driving habit. If you drive mostly hiway, you are going to get a lot more miles out of the rotor because you just aren't using the brakes that often. Where as if you do mostly city driving, you are almost constantly using your brakes. The type of brake pads matter to. If you are using an aggressive pad, it will eat more of the rotor. If you use a pad that gives you a lot of rotor life, than it's really not that aggressive of a pad. The condition of your calipers also play a role. If your calipers stick you will have more more wear (rotor and pad). The quality of your mechanic will also play a role because the more he has to take off when turning the rotor, the less life you get out of the rotor.
Also keep in mind that rotors of today are not designed the same as yesterday. They are usually thinner thus they don't last as long as days of old.