How often should the spark plugs be changed on a Ford Transit Custom?

I recently bought a Ford Transit Custom and I'm wondering how often should I change the spark plugs? I've heard mixed opinions and I'm not sure what is the best course of action.

It is recommended to change the spark plugs every 30,000 miles or every 3 years, whichever comes first. Neglecting to change them can lead to decreased fuel efficiency and potential engine damage. I learned this the hard way.

I drive my Ford Transit Custom for work and I change the spark plugs every 20,000 miles. It might be more frequent than what the manual says, but it keeps my van running smoothly.

I know it can be tempting to extend the spark plug change interval, especially when trying to save money. But trust me, it's better to follow the recommended schedule to avoid any problems down the road.