I assume that backups are incremental except for a new software version, i.e. it updates just the changes as a backup has usually taken about 15 mins, at a guess. When trying to do an IOS9 backup to iCloud, the iPad keeps telling me that it will complete in 3 hours. I stopped the backup a couple of times yesterday, but left it going all morning today. It now says that it has a backup on iCloud, but that 'the last backup could not be completed', under 'Back Up Now'. Both the backup on iCloud and the Back Up Now have times of 11:07am. Not sure what to believe here, so I am going to delete the iCloud backup; do an iTunes complete backup to a PC and then try again for an iCloud backup over night.