This is a good question!
It depends on when you saved the interval setting. Currently, the interval run time is completely separate from any other warm-up and is scheduled to run at the time you saved the interval setting. If you save the interval to 60 at 6:02, it will be scheduled to run at 7:02, 8:02 if no other warming is currently running (otherwise – skip; as the plugin can only run 1 warming at a time). So if the scheduled one overlaps with the one currently running, it will be skipped until there are no more warmings.
I think I will add a table so you can see more clearly at what time the next interval run is scheduled and whether it was skipped or not, and maybe even a setting to move it. Because the current implementation of attaching the next interval run to the time the setting was saved is very crude and non-transparent.
P.S. Your question made me look into the code of the plugin and I realized that the current interval functionality does not work properly. I will release the fix in ~1 hour. Please stay tuned and update the plugin on your sites when it arrives. Sorry for the inconvenience and let me know if you have any questions or suggestions.