I recommend an interval of about 18 months. Care and use of the Nikonos camera is included in the PADI Photography Specialty taught at the Brass Anchor. I replace all the o-rings in the body plus any lens, strobe or sync cord included. Battery is tested or replaced. Camera is cleaned and shutter and actuator mechanisms are tested, and lubricated.
Body is pressure tested.
All of the o-rings in the IV are the same as the V except for the camera back, which can no longer be replaced. N IV shutter and actuator are the same as the V, but electronics are not available. Do a final check for o-rings and caps in place before entering water. Rinse for 30 minutes in distilled or fresh water after use in salt water. Do not let camera dry with salt contamination.