I reckon you would get 4.5 hours out of the lamp on a full charge, maybe more.
I got 3.5hrs out of it the other night when working in the van and as straydog mentioned I was down to 2 bars. I was also on 'half light' although the difference between the 2 isn't really noticeable to be honest. The "light" it gives out is amazing, and there is so much light that you don't get shadows if the light is behind you when youre working.
A genuinely brilliant buy in my opinion and even better because it can be recharged in the van. It wasnt cheap but it was worth it