Reportedly, 20 percent of all accidents result from impaired vision caused by a dirty or damp windshield. At any given time, industry estimates conclude that 50 percent of the windshield wiper blades on operational vehicles are beyond their functional lifespan. In general, wiper blades should be inspected every six months and replaced as soon as they diminish wet-windshield visibility. Vehicle manufacturers typically recommend replacing blades every 12-18 months. Fresh blades are such vital safety components that many European countries have laws that require replacing wiper blades every 12 months.
You have two choices: replace the entire blade structure or just the rubber wiping part with a "refill." Depending on style, entire blade assemblies can be affordable, and they're a lot easier for the do-it-yourselfer to replace than refills. Blade units also prevent against inadvertently installing the wrong-sized refill.
Most blades are sold individually, but it's advisable to replace both at the same time (unless one becomes damaged before its useful lifespan is over). For most passenger cars, blade length is between 16 and 21 inches. Newer vehicles sometime have different-length blades on each side, so consult the application book at the parts store or the counter person. Even better, take along the old blades to ensure proper replacement sizes. If you're lucky, someone at the parts store might even install the new blades in the parking lot for you. It only takes a few seconds.