VolumeRetention in Bacula defines how long Bacula will retain a full tape for.
Once Bacula opens a tape it will continue writing (appending) to it until it is full (unless you have told it to only put one backup on a tape, in which case the tape will be considered "full" when the backup finishes).
A full tape starts the VolumeRetention clock -- Bacula will not recycle and reuse that tape until the VolumeRetention period has has elapsed.
For example, if you fill a tape on Monday at 08:00, and have
VolumeRetention set to 3 days the tape will become eligible for recycling on Thursday at 08:01 (3 days from when it was filled).
See the Bacula manual section on Automatic Volume Recycling for more details.