how often does my iMac back up to the iCloud

how often does my iMac back up to the iCloud

It doesn't. This is an ability for iOS devices. Your Mac will use an external drive and Time Machine to create backups. See Back up your Mac with Time Machine - Apple Support

On your Mac, iCloud is a sync service. This makes content available on different devices, but if you delete a file on your mac, it will also be deleted on iCloud. If you want to backup your Mac regularly on an external drive (and this is recommended) use TimeMachine