elastic search update Service software release in AWS console
I requested the R20210426-P2 update on a Monday and it was completed on the next Saturday so roughly 6 days from request to actual update. It's also worth noting that the update does not show up in the Upgrade tab in the UI, it shows up in the Notifications tab with this:
Service software update R20210426-P2 completed.
[UPDATE 11 Jul 2021] I just proceeded with updates on two additional domains and the updates began within 15 minutes.