How frequent shall I backup

When deciding how often to take level 0 backups, a good rule of thumb is to take a
new level 0 backup whenever 20% or more of the data has changed. If the rate of
change to your database is predictable, then you can observe the size of your
incremental backups to determine when a new level 0 is appropriate. The following
SQL query determines the number of blocks written to an incremental level 1 backup
of each datafile with at least 20% of its blocks backed up:
SELECT FILE#, INCREMENTAL_LEVEL, COMPLETION_TIME,
BLOCKS, DATAFILE_BLOCKS
FROM V$BACKUP_DATAFILE
WHERE INCREMENTAL_LEVEL > 0
AND BLOCKS / DATAFILE_BLOCKS > .2
ORDER BY COMPLETION_TIME;