The Persist extension has been created for this very same reason. Here are the steps:
* Define your condition (i.e. the presence of query parameter on the landing page)
* Create a new variable within the Persist extension that will store the stage (under "Store in Cookie")
* Define your Duration (would be 30 days)
* You can then define whether you want to set a text value to the cookie you create in step 1 or persist a JS variable that you create beforehand
Once done, your load rule for the tag will be something like page=thankyou AND the presence of the persistence rule you created with the extension.
Hope this is helpful.
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