There are currently four official restart times for the servers - 2 am MST (Arizona), 8 am MST (Arizona), 2 pm MST (Arizona), and 8 pm MST (Arizona). (Click the links on the times to convert to your local time.) The server will restart randomly from 15 minutes before the time up to 15 minutes after the time and not necessarily on the hour.
While these are the scheduled times, it is entirely possible that there may be an error within the game running on the server that may cause an unscheduled restart. This is normal, especially with popular game servers.
Staff asks that if a server becomes unresponsive for longer than 10 minutes to contact them on the Discord.