If you don't use the tractor much in the winter, then I'd change the oil before every winter. The problem is that moisture condenses in the oil, and the oil byproducts can be corrosive. So it's not good to let it sit all winter with a buildup of that stuff. Using synthetic oil helps, doesn't breakdown as bad.
If you use the tractor fairly regularly all year and don't let it sit, then I'd go the full 100 hours before changing.
I am not sure how much I'll use it, I used it alot this winter, because we got alot of snow, I will be using it next winter to plow snow. and occasionally during the summer as needs arise. with my guess is using about 50 hrs a year is that ok to wait the 100 hrs? with using it to snow plow should I change oil in fall or spring?