I agree with PT1; OCI at year's end.
The real question is whether the oil is needing to be changed, right? Well, a UOA would answer that, including the corrosive issues with a TBN reading.
Unfortunately, the cost of a UOA, along with the difficulty in obtaining one from such a small sump capacity without greatly effecting the makeup volume, kind of makes it easier to just OCI once a year. Regarding taking the sample, to catch the sample mid-stream, you'd dump so much of the sump that you're 95% of the way to an OCI anyway. That being the case, why bother with the UOA; just OCI. Or, you could buy a sampler tube, but with such a small sump, how do you know if you're getting a good representative sample?
The cost of a full UOA test at Blackstone would be $32. What's a OCI cost? If you run an HDEO and a decent aftermarket filter, you could possibly do it for $15? Here it's just cheaper to OCI than to extend the drain interval past the 1 year mark. You'd likely break even at two years (a guestimate?).
I don't believe for a moment that oil goes bad at the 366th day, but if you can change it cheaper, it's peace of mind. Small-sump, low-use equipment is just flat easier to OCI than to sample and try to extend an oil drain intervals.