Generally It takes under 11 minutes on wired devices with a fast Internet connection. Approximately 10 minutes or less for the firmware to be downloaded, and about 1 minute for the device to reboot and assume normal functionality. It may take a few minutes longer for various elements of dashboard to update and begin displaying live data.
https://documentation.meraki.com/General_Administration/Firmware_Upgrades/Cisco_Meraki_Firmware_FAQ
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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