A general technique for estimating the time available for safe egress from a fire is presented. By introducing a qualitative and quantitative model of hazard development, the details of the technique are formulated for the room of fire origin problem. The inputs to the model are the area and ceiling height of the room, data from free burn tests of characteristic fuel assemblies likely to be found therein, the anticipated mode of fire detection, and a criterion for the onset of untenability. The output is an estimate of the length of time between detection of a fire and the onset of untenable conditions. Result of applying the estimation technique are presented and discussed.