How does Log Retention Setting Work?

When "Both" is selected for log retention options, is it an AND or an OR relation between "By size" and "By date"?

Answer: It is an OR. This essentially means which ever criteria is met first.

How often does Vault test the conditions (if the amount of days elapsed or the log size reached the limit)?

Answer: It runs every 1 hour, after the previous cleaning job has finishes. It would also run after HELPER service restart