Backup Cycles 
This section gives you recommendations on how to plan a backup cycle for your Oracle database. We recommend a backup cycle of at least 14 days, preferably 28 days. For more information about the tapes required, see Backup Media.
Recommended 28-Day Backup Cycle
The guidelines are as follows:
Perform a full online backup each working day.
If possible, perform a complete offline backup weekly (for example, at the weekend), or at least once in the cycle. If offline backups are not acceptable due to downtime, complete online backups are also good enough.
Back up the offline redo log files each working day and after every online and offline backup. Be sure to back up the offline redo log files twice on separate tapes.
To verify the process, you need to:
Verify backups for physical errors at least once in the cycle, preferably once a week
Verify the database for logical errors at least once in the cycle, preferably once a week
Keep the verified full offline backup from each cycle in long-term storage, replacing it with a new initialized tape in the pool.
Although not necessary you can perform additional backups after changes to the database structure and keep these tapes in long-term storage. You can do this after any of the following:
A data file is added
A data file is moved to a different location
A tablespace is created, dropped, or reorganized