The solution also works a bit different. The retention is not the same as with VBR. In VBR, when you take a backup, it contains a "snapshot" of the moment. With VBO, we only backup items that are within the retention range. So for example, when the retention is 2 years, every item in someone's mailbox that is younger than 2 years will be protected. Everything which is older (so imagine a received email 3 years ago) won't be in the backup as the item is older than 2 years.
So it cleans out the repository itself. So if you are going to choose days, then there won't be a lot of data in the backup.
Last but not least, it is possible to change the interval to days if you really want to. https://helpcenter.veeam.com/docs/vbo36 ... tml?ver=15
You can select a specific # of days (bottom option in the dropdown list)