Datadog strives to have as few maintenance windows as possible as we understand our customers rely on us for critical business services. We endeavor to schedule maintenance windows with a minimum of one week's advance notice and historically have maintained a window less than one hour, scheduled for times with lowest use (nights and weekends EST).
Such notices contain details regarding specific service(s) that may be affected. When a maintenance window is scheduled it's posted to our public Status Page, users may subscribe to receive updates over email, SMS, Twitter and other common channels. For reference, here's an example of one of our past events: http://status.datadoghq.com/incidents/ss8ttllt7dgy
Typically, maintenance windows only impact the availability of our web app while alerts and data processing continue to operate as expected. Other times it may be that "new metrics" will not appear until the window is complete but are later backfilled.