sounds like a simple question, but with my particular light it's not. It automatically steps down the brightness after a few mins at 740 lumens to protect itself from heat and again after 30 mins at 400 lumens, so the highest setting with continuous run is 140 lumens and that does over 7 hours. 40 lumens goes 27 hrs and 9 lumens goes 130 hrs. I can turn it back on to its high settings immediately after it steps down, it's just not recommended. if you get a bit bigger flashlight you can get a longer continuous run of high output. I sacrificed a bit of continuous performance time to get a smaller package. Or you can think of it as I bought a regular looking flashlight that has some super powers :D
I highly recommend candlepowerforums.com for research. There is a bewilderingly ginormous amount of info there and they seem very friendly. kinda the Comanche Club of flashlight forums, I guess. :) Among the things to consider when looking for a flashlight are battery type and number of batteries, throw distance (is it more spot or more flood?), LED temperature (not degrees, I mean the perceived color palate of the objects you're shining the light at), switch setup (where is the "on" switch, where is the "mode" switch if applicable, and how do they operate), cost (duh!), features (do you want a momentary switch? do you need the strobe effect?), etc. check out the "recommend me a light for..." forum.