How long has a fire been at flashover if it has not vented from a single window?

A fire that has not vented from a single window and has reached the flashover stage is typically observed to occur within a timeframe of 1 to 2 minutes after the conditions are right for ignition and burning. During this period, the heat and flames build up, leading to an intense environment where combustibles can ignite almost simultaneously. Flashover is characterized by a rapid spread of fire due to the high heat environment, but prior to this event, the fire is contained and does not escape through windows or other openings.

The mentioned timeframe reflects the common behavior of fires as they develop; they progress quickly in enclosed or semi-enclosed spaces, particularly when ventilation is limited. In contrast, longer durations, like 3 to 4 minutes or more, usually indicate that the fire has either vented or that significant heat loss has occurred, which would not typically be the case in a flashover situation contained to a single window. Therefore, 1 to 2 minutes is the correct answer for the context of a fire that has not vented.