Short answer. eBike brake pads last 500 to 1500 miles depending on rider weight, terrain, and rain exposure. Coastal riders and daily commuters usually replace pads every 6 to 12 months. Hill riders often need pads every 3 to 6 months.
Step by step
Check pad thickness monthly. New pads are 3 to 4 mm thick. Replace at 1 mm or when you see the metal backing plate.
Listen for squeal or grinding. Squeal is often glaze or contamination. Grinding is metal on metal, meaning pads are gone and you are damaging the rotor.
Feel the lever. Spongy or long throw brake levers often mean thin pads or a hydraulic system needing a bleed.
Track your mileage. If you know your typical pad life, schedule a swap before you feel it. Waiting until you hear metal costs you a rotor.