"The definition of 'response time' depends on the perspective from which one approaches the data. In the fire service, 'total' response time is usually measured from the time a call is received by the emergency communications center to the arrival of the first apparatus at the scene. For the public, the clock for response time begins when the public becomes aware there is an emergency incident occurring and the fire department is notified. In reality, however, the response time clock for fire suppression begins at the moment of fire ignition and continues until the fire is extinguished."
U.S. Fire Administration: http://www.usfa.dhs.gov/
Topical Fire Report Series (January 2006/Revised August 2006), v.5 no.7