It all comes down to what hardware this is running on, which you omitted. Upgrading Pub can take ~2 hours, subs (you can do them all at the same time if you choose) ~1.5 hour. UCXN 1.5 hours each and can run along CUCM upgrade.
Just thought I would add this reference to the great tips from my buddy Chris (+5)
You can upgrade the Subs in Parallel as Chris noted which will save significant down time
Upgrading a Cluster in Parallel
When you upgrade a cluster that is running a supported version of Cisco Unified Communications Manager 6.x to Cisco Unified Communications Manager 8.5(1), begin upgrading the first node first. You can begin upgrading subsequent nodes in parallel after the first node reaches a specified point in the upgrade.
During the upgrade of the first node, view the installation log, install_log_.log, by using the Software Installation/Upgrade window in Cisco Unified Communications Operating System Administration or the command line interface (CLI). You can begin the upgrade of the subsequent nodes after the following information displays in the log:
You can also use the CLI to search for the relevant information in the install log by following this procedure:
10/14/2008 09:52:14 upgrade_os.sh|PRODUCT_VERSION is 220.127.116.11000-97|<:INFO>
Step 3 When the file search command finds the PRODUCT_VERSION string in the install log, you can start the upgrade of the subsequent nodes.
Caution If you want to upgrade the subsequent nodes in parallel with the first node, do not choose the Reboot to upgraded partition on either first node or subsequent nodes while configuring the upgrade options. If selected, the first node may complete its upgrade and reboot while the subsequent nodes are upgrading, which causes the upgrade of the subsequent nodes to fail.
When you are ready to activate the new version, you must activate the new software on the first node before activating it on all other nodes.
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