Thanks again for the comments.

We actually proceeded with the patching yesterday and decided to have a full outage.

This is a list of the actual faiback times experienced when failong over to the patched node (SQL2008R2 SP2 CU11 to CU12): the first instance took 2 minutes

the second instance took 3 minutes 10 seconds

the third instance took 3 minutes

and the fourth instance took 1 min 30 seconds

Of this approx 30 seconds was the failover process itself.

Total impact to online systems is one outage of 30 seconds and a second outage of about 3 minutes about 3/4 hour later.

Regards,

Steve