The empirical data for tungsten light bulbs show that life is extended by a
number equal to 12th power of rated to applied voltage ratio, what in case
of 12 V rated lamp run at 5 V would be around 36 000 times. Considering that
flash light bulbs are run for high light output and a life of just few
hours, the ratio is smaller than 36 000, but still quite large.

Regards

Miroslav Pokorni