I change seasonally. Every year before mowing, which is the heavy use for my tractors. I only surpassed the hours in one year once in the last 12 years.
I change engine oil and filter, air filter, and fuel filter. Yucky acids build up(combustion byproducts), especially as moisture can build in the winter.
The hydro, I change at the hours listed in the manual. Coolant, and axle oil too.
Also, grease every zerk on the tractor every 10 hours.
For a new to me tracto(ie used)r, I change everything when I first get it.