how long can CU and SP be different across a SQL Server Always on Cluster nodes
Does Microsoft support running SQL Always On clusters for a short period of time on different CU and/or SP levels. for a week or a couple of days?


This is not an official statement but I got this from my MS friend.

It would be supported subject to condition both CU and SP are supported. But the risk and burden lies on you. I have worked with such environment for very short period, to be precise for 4 days and after that I made sure all the replicas are on same SP/CU level. If you really need it, I would suggest to try to limit this to couple of days to be on the safer side. 

The correct approach would be to first test the SP/CU on UAT and then if you are happy proceed on production. But sometimes due to constraints this may not be possible. If replicas are on different SP/CU failover may cause primary replica to be on node which is not at latest SP/CU and you may see issues which was fixed in recently released SP/CU so this is very risky. The solution here is to keep such environment as short as possible.