Good thread here on the subject (including some real-world timing captures towards the bottom):
http://community.netapp.com/t5/Data-ONTAP-Discussions/Sanitization-time/td-p/38843
Make sure you destroy all the aggregates and designate the drives as spares.
It may or may no be redundant at the end of sanitizing them, but dropping into the special boot menu and performing a 4a after the disk santization may not be a bad idea (depending on your level of concern about the sensitivity of the data that was on the drives).
For a completely paranoid approach, have an "assured destruction" service show up and shred/cook the drives at 2000˚F after doing all of the above.
Hope that helps,
Chris