Do I need to upgrade all the MDs in the same cluster to match with VMM? No , you can have the MM running a higher version than the MDs
What will be the time frame for upgrading the MDs how long the existing AP will take to boot up after the upgrade. Any doc reference from Aruba with recommended practices would be helpful. Is there anything like pre-download to have more control over device reload.
The upgrade time could vary based on your environment but typically campus Aps could take from 5-10 minutes (assuming you are not using Live Upgrade) but you could decrease the upgrade time by using AP Preload Image
I suggest following the release notes upgrade guidelines
https://www.arubanetworks.com/techdocs/ArubaOS/8.4.x.x/Default.htm#ReleaseNotes/Upgrade/Upgrading.htm%3FTocPath%3DUpgrade%2520Procedure%7C_____7Sent from Mail for Windows 10