Well, I highly doubt most people put enough mileage on them to go by mileage (obvious exceptions like Simp, etc.)
So a TIME based interval is usually a better option to use.
You will most likely not run the engine enough to get enough soot, crud, etc. into the oil. So think about the acidic break down of fuel, soot, etc. that makes it into the oil.
I would say, depending on a few things, yearly, or some longer interval based on every 1,2,3,4 years, based on a somewhat regular interval.
If you have an accurate hour meter, this is another method to use.
And for the pretty low cost, getting an oil sample done every once in a while, should tell you whether you need to be changing the oil more or less often.
Btw, if you start your engine to 'just let it warm up', all the time, you are going to want to change the oil more often then just letting it sit without starting it.
Personally, on the big stuff, I like to change the oils every year or two. Unless I am working them.