A futon's condition matters more than a calendar deadline. For use in Jersey, check support, cleanliness and how well you can keep it dry. The owner's three-to-four-year replacement reference comes from regular nightly use and care in Japan; it is not a guaranteed term for either model.
What should I notice as the filling settles?
Look at the areas that take pressure each night and whether the support still suits you. Turning, rotation and airing are normal care habits, but they cannot permanently reverse years of material compaction. Nominal new loft should not be treated as a thickness that will stay unchanged.
Does occasional use make it last a fixed number of extra years?
We cannot assign that figure from frequency alone. Storage conditions and care still affect the bedding. Inspect occasional guest bedding before use, and do not keep a mouldy futon simply because it has received less use.