Is the customer notified of the next date of release?
By default, the notification is sent to the super admin customer account created as part of initial provisioning. If you wish to add more people to this notification list, please reach out to your CSM and place a request. They will internally reach out to the team which manages the list and post that all such people will start to receive build upgrade notifications.
How much time does the customer get to test drive the new release and adapt his environment in case of regressions, before accepting to go live with it ?
ACS is a SaaS, so the build upgrade cannot be stopped. It will be performed by Adobe as per the schedule. Test instances are upgraded at least a week before production and hence you can test for issues and report any business critical anomalies with the correct priority. Adobe Support will then test to identify if the problem is related to product and will promote it to Engineering. Depending upon the severity of issue the fix might be made available in the next major build or a patch release can be issued.
Can we assume that minor releases are transparent to the customer ? What about major releases ?
Major releases will be accompanied with a Release Notes update on the official documentation website. Minor releases are applied automatically and there is no notification issued. For more information you can liaise with Customer Success Manager or if you have bought Adobe Premium Services, you can raise your queries/concerns with the TAM.