The brake sensor wears at the same rate as the brake pad. A new sensor pretty much has the same thickness as the pad. When the pad wears down, so does the sensor. Once the pad has worn down past the BMW-specified threshold, the sensor would then triggers idrive in the car for the brake service alert to come up.
If your brake service light is not lit, that means your pad has not worn down past the service threshold. If you change the pads now, you may keep your old sensor to save some money. Then, your idrive estimate will remain at its current mileage until your new pads have worn past the thickness of your old sensor.