It depends on the gain, volume as well as what kind of load you are running the amp to. Low impedance (such as CIEM) or high impedance (such as >250ohm) load tend to suck out more power as the former demands more current while the later demands more voltage. I have ran E12 over 12 hours on two different times on typical 32ohm load on normal volume myself.
I am not implying BH is bad in anyway - but law of physics stated that you can't spend more energy than you have. The reason FiiO can't put 100 hours of battery life (i.e. E7) into E12 is because E12 is outputting a lot more power than almost any portable amp in the market now, and those power needs to come from somewhere. The only solution is to cut down power (which voids the design goal of a portable that is capable of driving some of the hardest load) or increase the battery size (which voids the 'portable' part of the amp). If you are getting 100 hours of battery life, then the logical question is, how is it possible to get such an exceptional battery life without compromising other aspect in the design? It is optimized for battery life or driving power? It is not an issue of right or wrong, but the designer's goal and how well it performs to the demand.