The frequency of replacing brake pads on a motorcycle can vary depending on several factors, including the type of motorcycle, riding conditions, and personal riding style. However, as a general guideline, brake pads should be inspected regularly, and if they are worn down to a thickness of around 3mm or less, they should be replaced.
Front and rear brake pads can be replaced separately without any problems. In fact, it's common for the front and rear brake pads to wear at different rates due to variations in braking force distribution. When replacing brake pads, it's a good practice to replace both the front and rear pads if they are worn out, but it's not necessary to replace them at the same time. You can replace them individually as needed.
It's important to follow the manufacturer's recommendations regarding brake pad replacement intervals and specifications, which can be found in the motorcycle's owner's manual. Additionally, if you're unsure about the condition of your brake pads or how to replace them properly, it's recommended to consult a qualified mechanic or refer to the manufacturer's instructions.