backup retention with incrementals

Please read through the "Schedule" section of the NetBackup AdminGuide I. It explains in detail how frequency and retention levels can be planned.

Here is an extract from NBU 7 Administrator's Guide for Windows, I  http://www.symantec.com/docs/TECH127079  (please read the entire section for more clarity):

Precautions for assigning retention periods
For full backups, specify a time period that is longer than the frequency setting
for the schedule. (The frequency is how often the backup runs). For example, if
the frequency for a full backup is one week, specify a retention period of two to
four weeks. Two to four weeks provides enough of a margin to ensure that the
current full backup does not expire before the next full backup occurs.
For cumulative incremental backups, specify a time period that is longer than the
frequency setting for the schedule. For example, if the frequency setting is one
day, specify a retention period of one week. One week provides enough of a margin
to ensure that the current cumulative-incremental backup does not expire before
the next successful one occurs. A complete restore requires the previous full
backup plus the most recent cumulative-incremental backup.
For differential incremental backups, specify a time period that is longer than the
period between full backups. For example, if full backups occur weekly, save the
incremental backups for two weeks.
A complete restore requires the previous full backup plus all subsequent
incremental backups.
■ NetBackup does not track backups after the retention period expires. Assign
an adequate retention period as recovering files after the retention period
expires is difficult or impossible.
■ Within a policy, assign a longer retention period to full backups than to
incremental backups. It may not be possible to restore all the files if the full
backup expires before the incremental backups.

PLEASE note the reference to 'Within a policy' - full and incremental schedules must be in the SAME policy.