My set of OEM manufacturer's documentation on the ZF Hurth 63 IV appears to be complete, with repair, installation, parts, technical data and specification.
None of those documents include the suggested fluid and filter change intervals.
The documents do categorize the usage of this transmission in our boats at the lightest severity usage, Pleasure Duty, 500 hours/year, plaining hull and large speed / power variations. There is also Light Duty, Medium Duty and Continuous Duty application categories for this transmission.
IMO the biggest risks are natural environmental and usage fluid contamination and raw water contamination from cooler failure.
An annual fluid / filter change would ensure this transmission would outlast the boat.
On my boat I designed and installed a custom high volume high pressure five zone oil change system, with zones to suck the transmission fluid from the bottom, where the drain plug was located.
IMO, the oil filter has exposed heavy duty reinforced pleats that look "bullet proof" and is reasonable to conclude that the filter could be removed and cleaned with a garden hose, permitted to dry and returned to service. I haven't done that refurbishment, but it looks possible to me.
https://live.staticflickr.com/4602/38697255400_870a0b6402_b.jpg