How often should i change my windshield wipers?

How often should i change my windshield wipers?

i normally buy them whenever i hear the streaky noise or when there are smudges after i use them

I usually change my windshield wipers every year just as winter starts because the long hot summer days are hard on them.

Thanks for the information about RainX. I got my cars' oil change about a month ago. They check for everything, including windsheild wipers.My car is a 2009,and I have to check the wipers more often because the heat of the summer down where I live , makes the wiper blades break faster.