If the car is more than three years old then don't bother. Just service it as and when required which is at least once per year and reset the indicator. Manufacturer's recommended service intervals only apply when cars are new. As soon as they're no longer new common sense must prevail.

It can track the standard 1 year service or the long life service 20000 mile, which ever it has been set for. If yours is on the 12 month then you cant change it it needs the garage to set it for the longlife service, but you can reset the 12 month service back to zreo yourself.