If you define only 7 days you will be fine if your routine (Incrementals and synthetic full) perform normally. But when your synthetic fails for some reason you are left with Incrementals only. You need 1 cycle defined in order to ensure a full cycle in each scenario

This attachment should help planning some changes. 

You can also make retention changes and then run the media forecast report against that copy to see what changes. Until data aging runs, the changes do not lock in.

You need at least 1 full backup to stay on disk in order for the subsequent incrementals to be coherent.

The number of cycles mandates the number of fulls available.

Essentially dropping to 0 cycles would mean that a restore cannot be guaranteed. 

You are better off with a selective copy that maintains the cycle requirements. Than a full copy that is potentially incoherent. 

A good way to think of aging rules is that the BASIC DAYS count starts on the last Incremental of a given Full.  If you never run a new full, then there’s no real start/end date to calculate.

The BASIC CYCLE count is only specifying the number of newer Fulls for that subclient that need to exist to satisfy the Retention settings, though that does not negate the above statement.

What @christopherlecky said is VERY valid.  If backups stop running and you notice 15 days later, you may end up with NO BACKUPS at all.  Cycles of retention exist to protect you from these scenarios.

Having seen people do this in the field, do NOT use 0 cycles retention.

As mentioned, this removes your safety net.  Yes, I have seen people not have backups available because they were using 0 cycles and discovered a backup issue too late.

Thanks,
Scott
 

i understand and i agree, its better to buy more data than reduce the cycle to 0 .. safety 1st :) 

You don’t necessarily need to buy more data, i would argue that if you have gotten to point of a cloud recovery you probably don’t need absolutely everything in the copy. 
 

cloud storage ain't cheap so just dumping absolutely everything in the cloud isn’t feasible for alot of companies.

Metallic is really cheap, we are paying 11€/month/TB :)