Successful sampling is as much a factor of thoroughness as it is of time.

Samples from a charred timber floor or from the surface of concrete may not provide positive results after a substantial fire, but those same materials may still have detectable quantities of ILR deeper in the substrate. Liquids can penetrate quite deeply into wood grain or into porous concrete, for example.

However, as John says, this is greatly dependant on the material, period of exposure to the ignitable liquid, and a host of other variables that are impossible to account for.