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I'm assuming you changed keywords in the content of your site, as opposed to changing the meta keywords tag, or changing the keywords you are tracking in Moz Analytics, but want to check to be sure. There is a lot of "it depends" here. If you're a big site like Amazon and change your home page, the search engines will notice that right away. If you have a five-page site that hasn't been changed in a year, the search engines might not notice you've changed things for quite a while.