Hi Isla, 

Generally, the manufacturer of a car sets recommendations and it's advised to follow them. If any unusual noise at all comes from a belt then it's probably already causing damage so there is no way for the average person to know when a belt should be changed. A mechanic can remove the top engine cover and examine the belt's condition, but that's rarely a five-minute job in itself.

I've looked up the recommended schedule for your car and apparently the belt should be checked by a mechanic at 240,000km - regardless of time - and then every 30,000km after than until it's deemed necessary to change it. We'd suggest erring on the side of caution and doing it earlier than that, but it's clear that it's ok to go by mileage rather than time for this engine.