the "contamination", or removal of the dirt from the engine is normal. what you should be worried about is the absorbtion of water from the atmosphere. this holds true especially for cars that are hardly used. the oil will tend to become less viscous and will not perform as well once there is water contamination that has happened.

re: the duration, longer is ok as long as the oil can take it. replacing it too often is a waste of money. just make sure the level is always in check.