The persistency is dependent on a number of factors - temp of course is one, type of surface and soil, strength of the agent and a few other factors. When they say ot will be gone in 3 to 8 hours the only thing that would make sense to me would be it is no longer at the lethal dose/concentration 50. Also we keep thinking the sarin in question is the same sarin we have always known. But an thickener or petro chem can be added to increase persistency. Also I worked a former chem site (Dugway) that had a few sarin filed items. This was many years after they had been opened, we put a plastic sheet over the bombets and let then set about a half hour, when we used the M 18 kit it turned positive for sarin. All the military docs I have on nerve agents differ with the web site posted.