I have had 2 of those lights. I lost the first one within a week. Make sure you tighten the rubber mounting band tight in the slots on the side of the light.
With the second one I found it was flat everytime I went to use it. I then charged it up and pulled it apart. The battery is a 520mAh battery. On the bright flashing modes I only ever got about 2 hours use before it went flat, due to the small battery and large drain.
I also found that the light is actually pulling 3 to 4mA all the time when it is seemingly switched off. The on switch is really just a mode switch with no off. This means if you don't keep charging it will likely be flat before you use it. A
I have removed the internal battery from mine and instead power it with a 3000mAh external 18650 battery that I can disconnect when not in use