Nikon D6 Reference Manual page 1134

D
Servicing the Camera and Accessories
The camera is a precision device and requires regular servicing; Nikon
recommends that the camera be inspected once every one to two years,
and that it be serviced once every three to five years (note that fees apply
to these services).
•
Frequent inspection and servicing are particularly recommended if the
camera is used professionally.
•
Any accessories regularly used with the camera, such as lenses or
optional flash units, should be included when the camera is inspected
or serviced.
1082 Caring for the Camera