From what I've read, alkaline batts deplete their charge in a non-linear curve, as opposed to the old-fashioned carbon batts that were linear in how they discharged. So the hours it takes from, say 9.2v to deplete down to 8.8 (which is still plenty healthy, btw!) is likely going to be longer than from, say, 8.0 down to 7.6v.
But to the point of your question, I've never measured a dead battery from my ES ...it's happened to me only once since I replace them proactively as a matter of course. But the lowest I've measured one of my still-working batts it was in the mid-7 volt range, if I recall correctly. Maybe someone else has gone lower? Hope that gives you some idea
Edward
