Disk queue lengths should not exceed 2 (per disk), alhough if they are sitting behind a RAID array (whether that be an array connected to an onboard HBA or connected through iSCSI) you should take into account the amount of disks and there role in the array - for example:
A RAID 5 array might be setup with 3 disks - with parity being spread accross all disks - but data is striped accross all disks and hence they are all considered data holders. This means when a VM is hosted on a disk configured on this array we must multiply the limit for one disk (2) against the number of disks in the array (3) - hence are acceptable disk queue threshold is 6 (2 * 3)
It is also important to take into account the I/O speeds you are getting from your disks with a tool such as FIO.
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