Keep in mind that Duplicati only stores changes every time it does a backup, so you probably can have many, many versions without it necessarily taking a huge amount of back end storage. Obviously this will vary depending on how much data changes between backups on your system, the type of data, etc.
One of my backup sets has 227 versions that span more than 3 years. Source side is 28GB and backend size is 88GB.
The “smart” retention is basically a custom retention policy of 1W:1D,4W:1W,12M:1M. After a backup is completed, the retention policy is evaluated to decide what backup versions to delete. This particular policy basically says:
- In the most recent 1 week, keep only 1 backup version per day.
- Then, In the most recent 4 weeks, keep only 1 backup per week.
- Then, In the most recent 12 months, keep only 1 backup per month.
This policy would delete any backup version over 12 months. Personally I don’t use it and instead use my own retention policy of: 7D:0s,3M:1D,2Y:1W,99Y:1M