Light & Motion has an interesting 2014 headlight test on its website, allowing you to compare various major brand lights. Shows output over time. Interestingly, the Niterider Lumina 650 I have for commuting (helmet mounted) drops off in lumen output very quickly, whereas others drop more slowly. Says my light starts out at 600 lumens and drops to 400 lumens after an hour. That approximates my experience.
http://www.lightandmotion.com/bike/urban700.html