how frequent is the icloud backup? can I retrieve a old backup?

It is my understanding that iCloud stores your two most recent backup. If you backup every day, today's backup will be the current backup and the backup from the day before will be your other backup. When you backup tomorrow, today’s backup becomes the second backup and the oldest backup is gone.

See this discussion and note the response from picas.

Restoring of a DELETED iCloud backup of an iPad

The backup is added to each night to iCloud if the device is charging.

There will only be one backup and you can't pick out data from it.