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|||When they get streaky you can try and use isopropyl alcohol and clean the silicates and dirt off them, that helps sometimes. If they still streak, replace them..... The Michelin blades are awesome for longevity and price vs performance. You can get them at W-mart.|||I usually change them once a year right before winter. Just because cold weather destroys wipers that have become dry and brittle over the summer. I buy the cheepies though.|||I usually change my windshield wipers every year just as winter starts because the long hot summer days are hard on them.|||Whenever they start to streak. If you can afford to get them buy Rainx Latitude blades. They are really good.|||when ever it dont do the job i got new ones 2 times this year