Your front bakes and your rear brakes are two completely different braking systems. Because of this you do not need to replace your front brake rotors and rear brake rotors at the same time. Most of your braking power comes from the front braking system. Different types of brake pads and how you ride all effect the longevity of the parts incorporated into your braking system. Best practice is to check your front and rear brakes a few times a year to make sure everything is in good working condition.