What constitutes harsh conditions? Excellent question, grasshopper. Obviously, bikes ridden hard under any conditions should have their oil changed more frequently, as should bikes ridden in extreme temperatures, dusty environments or with heavy loads. For instance, if you're pushing your air-cooled twin up the Rockies, carrying everything including your significant other (who brought along the kitchen sink) in midsummer, an oil change at half the factory-recommended mileage may prove beneficial. Conversely, if your bike is used for lots of short trips, particularly in cold weather, it may not get warm enough to burn off internal condensation. In such cases I'd also recommend halving the factory schedule. Look at it this way, oil is cheap and easy to change--engine parts are neither.