If the OP wants to know for sure then he'll have to do a used oil analysis.
However, if his only concern is getting through a season to some (relatively close) interval, then he'd have no problems going several more months.
Obvisously, as Jim stated, there is the presumption that all mechanical systems are in good repair. But, OTOH, if they were not, he wouldn't know otherwise unless he did a used oil analysis, which brings us back to the former point ...
I do go more than a year on some of my equipment with HDEO (1966 Mustang at 3 year OCIs; probably less than 750 miles total) and (Kubota tractor; currently starting into my second year of OCI). I sleep well at night, too.
OCI/chronological limits are fine IF you don't use used oil analysis. If you do use used oil analysis, then those limits are arbitrary and worthless. Oil does not go bad immediately at some date on the calendar. It does not suddenly fall back and punt at the 366th day. The shelf life for virgin oil is typically 5 years, and the use in the equipment is dependent upon severity of use, environment, lube construction, etc. But low mileage OCIs can easily go 2-3 years IF the mechanical systems are in good shape, and there a some used oil analysis out there (including Jim's and mine) to prove it.