If you run your machine in a lot of mud and water with the possibility of ingesting moisture, or in very dusty environments, you should change your oil often. Mileage is not a good way to calibrate ATV use. Hours of operation is better. If you're running in moderate to average riding conditions and moderate demand on the machine, once a year is enough with any decent quality HDEO. I don't worry about leaving the oil in over the winter. Did it for well over 25 years with my old Suzuki engines and never had a problem with them. I still do the same with my new machines. In the fall, I usually put my machines away after running them up to full operating temps. That pretty much ensures moisture is cooked out of the oil.
If the machine is seeing hard service, I'd be changing the oil at least monthly. If the machine is submerged sufficiently to stall the engine, an immediate oil/filter change is required as well as subsequent engine flush(s) to remove as much moisture from the oil as possible.
Oil management is all about common sense. Buy good quality oil suitable for the application, change it as deemed necessary to meet operating conditions. I prefer HDEOs. Great oil for very good prices. If the machine is to see winter service, go with a Synthetic, something like Rotella T6.