What is the usual replacement interval for brake rotors and pads in a 2014 320i?

Hello, there is no "usual" replacement interval. Brake pad wear is entirely related to driving conditions. If the vehicle is driven more "start and stop" traffic style, or in hilly areas -- brakes will wear faster. If the vehicle is driven on long highway trips with infrequent braking, brakes will wear slower. When brake pads are worn only on the "inner" side, typically the brake caliper slides are in need of lubrication. Brake rotors typically need replaced on every brake job on BMW's. Also, you'll need to replace the sensors. Brake wear is what it is. If the pads are low, they need to be replaced as a set -- whether one, two or all pads are worn.