I suspect that the DB in question is in "no-archivelog" mode If you will be running an offline backup. This is good as it eliminates the need to use any logs for recovery purposes. ;)
If you are using tape, one easy way would be to run the job with use new media (so it grabs a new tape, and then also mark media full after the job completes. Now you can keep that tape for as long as you need the backup. (Write protected, sitting on a shelf) As long at the tape does not get re-used, your data will be safe on that tape -- even if it ages)
If you are using disk storage (disk-library) then after running the backup, you can "manually pin" the job in the policy/copy. It will not get pruned until you "un-pin" the job.
This can be done by right clicking on the job from the copy (in the policy) and select "retain job". You will get a pop up menu where you can select infinite retention or retain until (user selected date).
Let us know if this helps.
Ernst F. Graeler
Senior Engineer III