The engine manual will usually recommend oil change at x hours or yearly. I would probably do it at the end of summer when you winterise the boat.

Here is a Yamaha maintenance schedule (from the USA), as am example.

https://yamahaoutboards.com/getmedia/67f82554-8a76-4f5d-9599-b982aa19afde/2009_Maintenance_Charts.aspx

Beyond of the initial 20hr / 3mo service, it calls for oil changes (both engine and gear oil) every 100hr / 1 year.

When both a use related metric and a time based metric is called for, it is whichever is less. In short, the fluids can degrade from both use and age.

I assume you're talking about 4 stroke outboard. Ideally you'd change it yearly before winter regardless of how little you use it.

Remember outboards are in a tough environment, regardless in what weather you go out in. Consider things such as water in your oil etc.

Also, don't forget about the gearbox oil!

Apparently oil has detergent in it that breaks down over time so even if you do 10 hours / year you should do an annual oil change.

John 

thanks will do