The instructions say replace the actual plug in bit after 6 months of continuous use or after the use of 6 refills including the original one.
JS, I bought two for the boys when we moved and only really need one plug in unit so I'd happily send you a diffuser - it's just the beige plug in bit, no liquid as I used that when we moved but it has only been used once. PM me your address if you want me to send it.