If this is your first backup it can take a while to finish, but a rough estimate would be about 50 minutes per GB. Unless you had a massively large backup it should not take 8 hours. Also, subsequent backups will be much faster as it only backs up changed files since the last backup.
A failure to back up may be caused by a corrupt existing backup that needs to be deleted, or by data on your phone that is preventing the backup. To troubleshoot these, try deleting your last backup (if you have one) by turning off iCloud Backup in Settings>iCloud>Storage & Backup, then tap Manage Storage, swipe across your backup and tap Delete, then go back and turn iCloud Backup back on again.
If it still won't back up, you may have an app that is preventing the backup from succeeding. To locate which one, go to Settings>iCloud>Storage & Backup>Manage Storage, tap the name of your device under Backups, under Backup Options tap Show All Apps, then turn them all to Off (including camera roll) and try backing up again. If it succeeds, then the camera roll and/or one of your apps is preventing the backup and you'll have to located by process of elimination. Turn the camera roll On and try backing up again. If it succeeds, turn some of your apps to On and try backing up again. If it succeeds again, turn some more apps to On then try again; repeat this process until it fails. Eventually you'll be able to locate the problem app and exclude it from your backup.
If it still fails you may be having issues connecting to the iCloud server. Solutions to that include power-cycling your wireless router (unplugging it for 15 seconds, then plugging it back in), resetting network settings on your iPad (in Settings>General>Reset), updating the firmware on your router, and changing the router's DNS settings to use Google's public DNS (see https://developers.google.com/speed/public-dns/docs/using).