The issues with large idle times are moisture/contaminate settling. If the tractor is stored indoors it greatly reduces the moisture. If the machine is started and run often, it will circulate the fluids. Both will negate the negatives effects of time. If you do both I wouldn't worry unless you get outside of 3 years. Now if the machine sits in the rain, and gets started once a year, then ya you need to change it at the factory interval for time on engine oil. Which tends to be a year. All of this assuming inside of the hour change interval. The engine oil sees more combustion/fuel contaminations, compared to the hydraulic fluid. The hydraulic fluid is really only affected by hours of shear strain(use), and the moisture content since the filters removes suspended contaminates. The fluid becomes very cloudy when moisture content becomes an issue. Once the hydro fluid starts taking on a cloudy or burnt appearance it has reached the end of its additive ability. The fluid should be changed regardless of the run hours.