The light will run for up to 7 hours on high output, but it automatically drops to low-power mode for a couple more hours riding so that you aren't left in the dark.
The light pattern is quite narrow, so you won't get much visibility beyond the road directly in front of you.
Dan Joyce of the CTC gave the light a review in the club's Oct-Nov 2010 Cycle magazine, with a note that more details are available from cyclelife.com. Unfortunately that website has no mention of the light or even Asteri. However, Merlin Cycles has it listed for £47: RSP Asteri 3 watt rechargeable front light, as do AshCycles: RSP Asteri 3 front light [LAA910].