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 Managed File Transfer 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