I've currently set the backup policies to have a retention period (the 
duration for which you wish to keep the backup data) of 1 year, and a 
rotation period (the rate at which you wish to move tapes from the library 
to the storage facility) of one week.  Both full backups and incremental 
backups have a one-year retention period, as day-to-day changes in files 
are likely critical to this company.  As such the incremental backups must 
be kept in addition to the full backups for as long as you need the data.  
I've found that a one-year retention period is a good general-purpose 
timeframe, as it offers a good balance between data security and overhead 
cost (in terms of the number of backup tapes one would need to purchase).  
However, feel free to change this retention period to better suit your 
needs.  For example, some data, such as tax and other financial records, 
have legal requirements for retention.  Other data, such as preliminary 
documents, can probably be expired when the final version is complete.

The rotation period is weekly, as a backup tape or tapes for a given 
policy will typically fill up at about that rate.  Given the number of 
tapes in the library, this gives you about a 1-2 week grace period 
before backups begin to fail.  It has also been determined that the data 
is important enough to move off site for safe storage at this rate.

IMPORTANT: Log all changes to retention and storage policies in the legal 
doc!