Check here for an explanation of the immutability period for Azure - https://helpcenter.veeam.com/docs/vbazure/guide/immutability.html?ver=60

Also here for SOBR capacity tier - https://helpcenter.veeam.com/docs/backup/vsphere/immutability_capacity_tier.html?ver=120

You have to take in to account the GFS flag based on your retention settings.

Please ensure “Do not enable immutability for existing containers in the Azure portal”.

KB4416: How to Configure Azure Storage Account to Leverage Immutability with Veeam Backup & Replication

They have updated last month as it is long period stays if we enable immutability lesser or longer it seems. 

https://helpcenter.veeam.com/docs/backup/hyperv/hiw_immutability_os.html?ver=120

Actual retention = job retention policy + immutability period + block generation period (10 days)

For example, the backup job is created with the following settings:

• The job retention policy is set to 30 days.
• The immutability period is set to 14 days immutability.

• Block Generation is 10 days.

According to the backup job retention, an object storage repository will keep backups for 30 days. The immutability period adds extra 14 restore points as a delta that prolongs the job retention policy. Plus, 10 days from the block generation are on top. Thus, you need to plan storage that will be able to keep your backups for 30 + 14 + 10 days = 54 days.

This is huge, but in real scenario which is working differently in my test lab and in the production. It is confusing and no reporting options as well. 

Thank you so much Cary sharing this info. We have followed the same article only while configuring them.

Well that help page is correct and you would keep the restore for 54 days.  The other option would be to have a lower retention for the job and use the GFS points for longer.