How does the app handle large volumes?
The app is tested against 500k tickets, to be deleted by one or more tasks. The time it takes to delete all tickets depends on the load of your account (we have to respect Zendesk's API rate limits).
Single bulk delete
In a production/live Zendesk set-up it took about 2 weeks (non-stop) to delete all tickets and users.
Based on the single bulk delete, we've estimated that the app can delete about 35k tickets each 24 hours. That should be more than enough for all you incoming requests.