Typically every fall when I put the atv's away for their long winter nap, I change oil. However, this year there are only 2 easy rides on them, totaling probably about 70 miles max.
It all depends on the oil you use, and the temps where they are stored.
Your oil should have a good corrosion inhibitor component in it...especially if stored cold... oil can go acidic if the right conditions happen.
Good way to store those things is to drain all the gas out... fill it up with AV gas and start them every few weeks... and let them warm up.... the AV gas has a very long shelf life and does not have any of the ethanol that causes problems in storage.
Some people never change the oil, I would say with 2 rides I myself wouldn't worry about it. If your worried just change it, I have never seen oil go bad in any type of engine as long as it wasn't contaminated.