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Thanks so much for the follow up question! I don’t have a specific frequency or timeline I can provide however, you can use Mozcast (http://mozcast.com/) as a resource to help determine how turbulent Google SERPS are for any given day. As far as changing your keywords in your Campaign, that would really be up to your discretion. Keep in mind though, that any time you add keywords to your Campaign, data won’t populate for those keywords until the following Campaign update.  If you add or remove keywords between each update, you may see significant fluctuations in your Search Visibility, too.