Naim CD Player & Amp Service Interval

How old does Naim CD player and amplifiers need to be before servicing in generally recommended.

I have a Naim CD5x, Naim 122x & 150x plus a Flatcap.

Also would they all need servicing at the same intervals. What would the service include and are there expected to be any audible benifits in sound quality or is it just to maintain the longevity and reliability of the product.

Finally, any idea of the approximate cost of doing this.

thanks