When exactly does Auto-Purge run?

Short answer:
Once per calendar day, the first time your site performs an eligible action (opening the Joomla Administrator area or running a CLI task).

What “once per day” means in practice:

• JMail Log uses your Joomla Global Configuration → Server → Time Zone to decide “today.”
• When an admin logs into the backend (or a CLI job runs) for the first time that day, Auto-Purge checks for old logs and deletes anything older than your set limit (default 365 days).
• It then records the date as last purge date.
• For the rest of that same local day, it won’t run again.

Safety note:
Nothing happens automatically at midnight. The cleanup is only triggered by that first admin/CLI activity of the day, and it never runs more than once per local day.