To send a data breach notification to all your users you can navigate to GDPR -> Tools -> Data Breach.
After you input all the required information an email confirmation will be sent to the data controller. On confirmation, we will begin sending bulk emails to all registered users. This can be a very long process if you have a lot of users. By default, the plugin will send 100 notification emails per hour. You can tweak this number in the plugin settings depending on your server outgoing email limits.
If you have 600 registered users and keep the hourly limit to 100, it will take around 6 hours to send all notifications.