50h at full power will never, ever be the case.
According to Petzl (
https://www.petzl.com/INT...), it keeps the 900lm at ReactiveLighting between 2 and 30h. I wouldn't expect more than 3.
The Nao keeps the 750lm up to 6.5h with ReactiveLighting. (
https://www.petzl.com/INT...) I can confirm this. You can wear it on your head for a night.
Whereby:
- With Reactive Lighting, all the lumens are rarely used.
- For most things, 200 - 300lm are enough. More is only relevant for night skiing, mountaineering, mountain biking or fast runs.