Change rate
Data change rate will determine how much new data is written each backup cycle. The best way to determine this is via the NetBackup Activity Monitor and examining the size of differential backups.
When sizing for long-term retention, that nature of the change is important to understand. Consider the case of a 5% daily change rate. When the month one backup is compared to the month two backup, the change between these backups can range from 5% (the case where the same data changes every day) to 100% (the case where different data changes every day). In most environments the former is more likely than the latter. There are cases for the latter, such as security video storage.