During the upgrade (if using CCA) there will be a backup, and upgrade, and a restore, and then a reload.

Looks like during the backup it remains available.  During the upgrade, restore, and reload it is unavailable (access to CUE says "unavailable, try again later" during the restore, but fast busy otherwise)

For maintenance window planning, approximately 20-30 minutes I think.  The CCA GUI will give you estimated times for each component as well, before you click UPGRADE.