Before I answer this question, we first need to discuss the types of “real-life” restores that you may need to perform:

1. File or folder restore (i.e. documents)
2. Bare-metal full system restore – in-place
       (i.e. to the original hardware, say after a RAID array failure)
3. Bare-metal full system restore – alternative platform
       (i.e. to different hardware or to a Virtual Machine (VM))

To perform a file or folder restore, the backupset (i.e. all the backup images) just needs to be intact.

Likewise, to perform an ‘in-place’ bare-metal full system restore, again the backupset just needs to be intact.

However, to perform a bare-metal full system restore to an ‘alternative platform’, not only does the backupset need to be intact; but you also need to have the appropriate drivers for the alternative platform.

So, to be able to perform any of the above restores, the backupset (that is all the backup images in the backupset) needs to be verified and confirmed as being intact.

And in addition, to perform a full system restore to an alternative platform other than the original / existing hardware that the server is currently running on; you need to have the necessary drivers for that alternative platform.

The other aspect that needs to be considered in determining the frequency and types of test restores, is the size of the backupset, that is how many images are in the backupset and the size of the backup images; as these two factors determine the time that is required to verify a backupset.

The number of images in the backupset determines the “backup window”.

Say, you are performing nightly backups, and your backupset currently has 90 images.

Then your “backup window” is 90 images or approximately 3 months. That is, you have the ability to restore either the full system or any file or folder to any point in time within those 90 images.

A popular “backup window” is up to 1 year (i.e. up to 365 images).

Depending on the size of the backup images in the backupset (for instance a file server will usually have larger images than an application server), it can take between 24 to 72 hours to verify a backupset with 300 images or more. 

So, taking all of the above factors into account, the answer to the question: