If the mains power failure lasts more than the grace period, the interruption in broadcasted to the user at the end of grace period. Otherwise, the user wont be disturbed.
This value can be modified with the advanced settings program from the window of MGE UPS properties.
For that, select the branch: "Advanced setting\Software settings\Control-module settings\Programming events and actions\Utility failure\Notification `Utility failure' " and change the value of the action parameter "Time delay" by double clicking on it.