Ideally, we can't really predict the duration of an upgrade as there are many factors contributing to itself - like, the NIOS version, the appliance type, database volume. While upgrade, what actually it does is that the NIOS creates a new disk partition, load the new code onto it, dump the database and then reboot itself to the new code. One thing I can say is that the actual service impact usually be between when the appliance goes for a reboot and joins back to the Grid succssfully. A reboot usually takes a few minutes, but may take more time during the upgrade.
To better optimize the upgrade, NIOS has an option to schedule it . This means, you can group the members, schedule a time , define it be simultaneously or sequentially etc. You may refer to the topic 'About upgrades' in the admin guide and also the release notes of 8.4.4 in order to understand the version / hardware compatibilites, new features etc.
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