One of my readers recently wrote to me for help with an alert message he had on his iPad. He was getting a notice that his iPad had not been backed up for 8 weeks. Your iPad will back up to iCloud automatically if all the following conditions are met. Your device must be:
1) plugged in to a wall outlet/power source.
2) connected to a Wi-Fi network.
3) locked and inactive.
These 3 conditions are generally met overnight while you are sleeping.
For more information on iCloud backups and alert messages, see this article from Apple.