There are far too many variables for a rule of thumb. It depends on the amount of IL used, the substrate, the amount and type of debris that's covering the location of the ILR, the temperature of the fire and the ambient temperature afterward. When poured on synthetic carpet, the carpet can melt and entrap the ILR indefinitely. Same for plastic containers. They never give up the ILR unless they are completely consumed.

Carbonized substrates tend to hold ILR for a very long time due to adsorption of the same kind that happens in the lab when the samples are analyzed.

My rule of thumb was always to collect as much as I could and test it no matter how long since the fire. You have to remember that the detection limit for ILR is incredibly low, as low as 1/500 of a drop (0.1uL). So always collect a comparison sample. You wouldn't want to have a positive sample that means nothing, would you? That's what will happen if the lab finds ILR but there is no comparison sample.

The best location is in the middle of the pattern, not at the edges, as was conventional wisdom for many years.

John J. Lentini, CFI
Scientific Fire Analysis
Islamorada, FL
www.firescientist.com
scientific.fire@yahoo.com