The recommended interval for changing the brake oil in a KTM Duke 390 or any motorcycle can vary depending on several factors, including the manufacturer's guidelines and your riding conditions. To ensure accurate information, it is best to consult the owner's manual specific to your motorcycle model.
In general, it is advisable to inspect the brake fluid regularly for any signs of contamination, discoloration, or a decrease in its performance. If you notice any of these issues, it is recommended to change the brake fluid regardless of the mileage or time since the previous change.
This interval is recommended to maintain optimal brake performance and ensure the hydraulic system operates smoothly. Keep in mind that this is a rough estimate, and it's essential to refer to the owner's manual for accurate information regarding your KTM Duke 390 model.