Hello,
These maintenance windows differ depending on what is being done.
The latest one last weekend was purely advisory as our systems were not being taken down (and in the end there was no disruption whatsoever).
In the instances where we do need to disable access to our systems, there could be any number of error messages or time out errors, again depending on what is being done, so its really difficult to provide a comprehensive answer unfortunately.
Apologies
SDN