I was affected by this 'scheduled maintenance' last night when trying to access my account on the My Verizon web site and the My Verizon mobile app. The My Verizon server was under maintenance was the message displayed. On the web site, Online Chat was available and I chatted with an agent to find out if there was an outage or what. They said it was a 'scheduled' maintenance, but said their notification did not contain any maintenance 'window' identifying how long the server would be down. As a former engineer, equipment/servers scheduled maintenances always had a window defined and timeframes identified/communicated.
It just means that they are proactively making repairs/upgrades rather than reacting to a service interruption. Scheduled is the opposite of unplanned. It doesn't mean there is actually a scheduled time for a service interruption.