The respawn time for large monsters occurs at 3-hour intervals, namely at 0:00, 3:00, 6:00, 9:00, 12:00, 15:00, 18:00, and 21:00 UTC time.
Adjusted for the Pacific time zone, that would be at 2:00 AM / 5:00 AM / 8:00 AM / 11:00 AM / 2:00 PM / 5:00 PM / 8:00 PM / 11:00 PM PDT. You can adjust that accordingly for Mountain Time, Central Time, and Eastern Time for the rest of the North America.