I've started reading the Bible at night, trying to make it a habit. I would use my PT Attitude to read at night before i went to sleep. I once left it on on accident one night, and then again on another night. THe second time i left it on, the light output the following night was less than desireable for reading with unadapted eyes. I've used the battery for MAYBE a total of 30-40 hours before i had to change it. So your observations are right...but i don't think i'd call it DEAD after 40 hours...just not useable. Keep in mind that i used this for reading and not finding my way around the house. THe light output at that point is no good for anything...not even finding your way through a bedroom.
I'd say you'd get high output for the first 5-10 hours, useable/readable light for 20 hours after that, and wasted light not useable for anything after 35-40 hours. Still pretty good for a light like this. I'll see if i can do some kind of test but if i did it would be purely objective, and not scientific. Maybe Craig would be better at doing this type of experiment.
P.S. Most LED lights out there are great, especially ones like the Inova X5, Eternalight, and the Princeton Tec Attitude. But the big problem i have with these lights...like all other lights, is the way they get weaker and weaker as battery drains. Of course there is no way of getting around this, but you CAN prolong it's brightness to some extent with regulation - like the line of ARC lights. I'm seriously considering the ARC AAA but am waiting for the LS to come out or the LE to be readily available again.