Working in support, I usually get asked how many days I should keep journals. Should it be two days or after two backups? More? Less? Why two?
The correct answer (for most of the environments) is that you should keep the journals since the last validated Backup. I.e., until you don't check if a Backup is valid (restoring the file and checking with the Integrity utility), you can't be sure there is a good copy of your data and can't purge the journals safely.