When you enable backups for a Droplet, you can choose between daily and weekly backups.
You can also customize the backup window, which is a 4-hour window of time during which the Droplet automatically initiates a new backup. For weekly backups, this also lets you choose a day of the week for your backups.
As an alternative, you can also use SnapShooter to automate taking snapshots of Droplets. SnapShooter offers a customizable backup frequency and retention policy.