In reply to Rob Greenwood - UKClimbing:
It's very hard to give an accurate figure for the burn time in reactive mode!
If you're out in very dark conditions with no other runners/etc and are constantly looking up/ahead then the burn time will be at the lower end of the scale, as mentioned a minimum of around 2 hours.
If you're out in conditions where you're with others, moving through built up/well lit areas and are maybe looking at your map, etc more then you'll see far higher burn time, the headlamp will dim itself to meet your need and give a longer burn time. e.g. We often see longer burn times running in snowy conditions as the headlamp reduces the power when looking at your feet more than in normal conditions.
A top tip for longer burn time is to angle the headlamp down more than usual, this will reduce the light output but you can still look up for the full beam.