Hi
Storage highly depends on your individual configuration.
There is no "one-size-fits-all"
Personally, I do not recommend offline storage, mainly for those reasons:
- Index rebuild (randomly reads from secondary storage)
- Offline Vault / Vault Cache enablement/rebuild (same)
Partitions, with EV8, can be rolled over based on time or size, or even both.
So you could create a new partition every year, over every 500 GB or whatever.
It's recommended to create a new partition every once in a while, mainly for backup reasons to split it up.
You may be interested in the Backup Whitepaper:
https://www-secure.symantec.com/connect/articles/enterprise-vault-best-practiceev-backups
Cheers