How long do windshield wiper blades last?

The typical answer from most industry experts is 6 months to a year. Having had vehicles in Texas, Florida and Washington I have found the right answer to be whenever the wipers are streaking on the windshield.

My first step is to simply use a damp cloth to wipe the blades clean. If that does not work you need new blades. 

Check out this article on the topic https://auto.howstuffworks.com/under-the-hood/car-part-longevity/windshield-wiper-blades-last.htm