you need to set this on the jobs themselves, enable the retention policy in the blasadmin config, and then run the blcli Delete command that enforces the cleanup.
there's a defect w/ the cleanup in 7.4+ I believe where it will not honor the retention time properly, however there is a hotfix for it. ask support about that.
ah..great..thx for the info. So, i need to set up a nightly job to execute blcli delete, correct?
yep. nsh script job should do it.