There is one ir.cron object that will work for a certain period of time. There you can specify the time when you want to send an email.
This object will call the function specified in the Method attribute. In this function, you should look for those invoices that are in the created state. Then check the date when it was created, and if its> = 7 days, then send mail.
Or
You can create ir.cron for a specific invoice workflow action that will have a Next Execution Date , like after 7 or 8 days.
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