It's recommended to change your Windshield Wiper Blades every 6 months because as you use your wiper blades the rubber on the wiper blades degrades and can make it hard for the wipers to keep the rain off your windshield.
Some wiper blades may last as long as a year depending on the climate and how often you use your wiper blades.
I always just put new wiper blades on my vehicles every 6 months because they are so cheap and I don't want to be driving in the rain or snow and have bad wiper blades.
I need to be able to see clearly out my windshield so it's a matter of safety and the wiper blades are easy to replace and cheap enough that it's worth it to me to replace them every 6 months which is what is recommended.
Some places such as Autozone will install new wiper blades for you free when you buy the wiper blades from them.