Marine gearbox oils have other additives in them , so I'm not sure about the colour. Most ordinary EP80/90 oils Ive used (including marine ones) are clear yellow ( if you follow me) and when most start to break down they will turn black, but also smell strongly like natural gas, in doing so.
When they get this way, they dont offer the correct lubricity and should be changed promptly. And as already mentioned, milky colours are water contamination
On land based products using gear oils, Ive noticed the speed of break down is relative to a number of factors, namely, the oil capacity of the box, the amount of gear surface area using the oil, and the loads placed upon them. I've not had any outboards that have been particularly harsh on ther gear oil, so I imagine they are loaded moderately or well enough designed to have the right amount of oil servicing that particular gear area.
That said .. Ive always done them every couple of years, so as to inspect for possible water ingress, and my use is only light leasure
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