I turned it on at 11:30 last night. When I got up at 7:30 it was noticeably dimmer, still useable but fading fast enough to see over the next half hour.
Say eight hours good, and that's with cheap Rayovac non-alkaline batteries(6 for $1 at the dollar store). With good alkaline batteries, I imagine it'll extend that a bit. Next time I get a chance I'll pick some up and give them a try.