A serpentine belt lasts for about five years or 50,000 miles. It is common for serpentine belts to last longer without any issues, but it's a safe practice to always swap them out as suggested.
Serpentine belts were designed as a replacement for the v-belt, which uses a multiple belt system. A serpentine belt is a single belt used to drive the pulleys on the alternator, water pump, air conditioning compressor and power steering pump. It is called a "serpentine belt" because it snakes through and grips each different pulley so that only one belt is required. The disadvantage of a single belt is that, if it happens to break or is knocked loose, all of the engine accessories that are attached are immediately unusable. Even though serpentine belts may last 50,000 miles or longer, they should be checked periodically, such as during normal oil or fluid changes.