The answer is that you should change your oil at least once a year. When you ride the bike, small amounts of combustion by-products work past the rings introducing, among other contaminants, water into the crankcase. This mixes with the oil and gets distributed through the engine where it can begin to corrode unseen but expensive bits. If the bike is used just for short trips and rarely has the opportunity to get and stay hot for a half hour or more, the moisture will build and dilute the oil, diminishing its ability to lubricate, clean and cool. In fact, if your bike is used almost exclusively for short rides and rarely runs fully warm for extended periods, you should change the oil maybe three times a year. Ideally, you should get your engine up to operating temperature and keep it there for a half hour at least twice a month.