Automated backups, I believe, are every few or couple of days. I haven't used DO's backups as I prefer my own way, but my guess is they overwrite old backups each time?

There is no rhyme or reason to the backups as far as I can tell. They run about every 3-4 days and DO keeps 5 backups. They seem to keep the latest 3 backups + 2 older ones.

For the time being, DO isn't charging for backups so you can't complain. I am also backing up my droplets on a more formal basis using BackupPC to location outside of the DO network.