The time required to upgrade the Manager depends on the size of your deployment and the size of your database. The upgrade process of the Manager itself may take an hour to complete.
Operating system upgrade downtime — The latest Manager 11.1 is supported on various operating systems as mentioned in Central Manager and Manager system requirements.
If you want to upgrade the operating system of your Manager server, you must factor this in when you estimate the Manager downtime.
How a Sensor functions during the upgrade downtime — While the Manager upgrades, the Sensor (which has not yet been upgraded, and which loses connectivity to the Manager during the upgrade) continues to inspect traffic and accumulate the latest alerts (up to 100,000 alerts) while the Manager is offline during the upgrade. Note that the Sensor sends these queued alerts to the Manager when it re-establishes connectivity with the Manager after the upgrade.