This is a more complicated question than other car and truck service issues and depends on how you drive. If the majority of your driving is done on the highway or on roads that require little braking, the number of kilometers you've accumulated isn't a good way to keep track of your brake wear. A better way to measure your brake wear is to look at your driving style: do you anticipate a stop ahead and reduce your speed or do you “ride the brake” to prepare for a stop?" This style of driving will mean you'll go through brake pads faster and require more servicing.
Check at a regular interval like you do for fluids. 35,000 kilometers is a good interval to use so long as no problem has developed such as road debris getting stuck between the pads and the rotors, or hardware failure in your brake callipers, which can also wear out brake pads faster than usual.
Get a feel for your pedal. If the pedal feels soft or travels too far to the floor or if it seems to take longer for the car to stop, you should have your brakes checked. On the opposite end, if the pedal is high and you hear a hissing sound, the problem could be a vacuum leak or the brake booster.
Listen for squeal or any kind of metallic noise. if you hear it, you can bet that it won't go away on its own. What's happening is the brake pad has worn out and the steel frame is contacting the disc brake rotor. Depending on how deeply the rotors are gouged, you will either have to resurface or even replace them.
Check for a fade or a pulse. Fading is when you press the brake pedal and it seems to take hold but then you feel a surge and a sensation that the brakes aren't holding. Pulsing is when there is intermittent back pressure on your foot when you apply the brakes which can be caused by distortion of the brake rotor.