Where do I find how often my iPhone or iPads backups

By default, iPhones and iPads backup nightly when connected to Wi-fi and power and the screen it turned off if the "Backup this iPhone / iPad" switch is turned on in Settings ➜ Account (top go Settings) ➜ iCloud ➜ iCloud Backup

There is no frequency setting for the backups. If it can backup it will backup.

You can verify when the last backup was made, in Settings ➜ Account (top go Settings) ➜ iCloud ➜ iCloud Backup ➜ Your device name ➜ Last Backup. It holds the date of the last backup it was able to perform successfully.