I do fire station modeling for a countywide fire department. Estimated drivetimes to an incident are a huge factor when deciding where to locate new fire stations. To get those numbers for decision makers, I create a Service Area in Network Analyst using our existing stations and any potential station changes the Chiefs are considering.
I create a new service area and set the default break to 20 (minutes) as this is higher than any actual drivetime that will be returned. The resulting polygons provide me with a Tobreak value that is the maximum modeled drivetime encountered in that service area (e.g. 5.231). In the last month or so, I have stopped getting the maximum modeled drivetime and instead the Tobreak field is just whatever Default Break parameter I set, regardless of the actual size of the service area.
I'm currently using ArcGIS Desktop 10.6