Exchange backup retention best practices

I'm fighting storage issues on our data domain after 8 months in operation. Some things I have identified like additional servers that weren't part of the initial assessment that helped with the increase. My highest offender in terms of change rates are my exchange backups. I wanted to see how everyone else is doing their exchange backups and how long they have their retention policy set for them. I think that's part of my problem with the storage is because I am effectively backing up exchange and emails in 3 different methods.

VM backup of one of the Exchange servers. 4 day retention and is 1.1 TB. 4 day retention.

DAG setup with the Exchange VSS plug-in backing up the passive data bases. 7-800 GB 31 day retention

Barracuda Message Archiver - Emails themselves - 1.6 TB 7 day retention.

I am thinking the VM backup of one of the Exchange severs I can get just get completely taken out of Avamar entirely. I also think that the 31 day retention on the DAG is high. Working with the SQL admin for a similar space issue with his stuff in Avamar he was saying that his SQL database backups are worthless after 2 weeks because of the constant changes to them.

I wanted to see how everyone has their configured since my work is still new to Avamar and working on fine tuning so I have enough storage while my work looks into another Data Domain Shelf or two.