That's interesting. One hour is not normal. 25 watts will drain the battery in about the same time with a 1.5 ohm setup as with a 0.5 ohm setup.
I disagree. Even at the same wattage, you're drawing much more current from the battery at lower ohms.
25watts at 1.5ohms:
V*V/1.5 = 25
V*V = 25*1.5 = 37.5
V = sqrt(37.5) = 6.123
Amps = Volts/Ohms = 6.123/1.5 = 4.08
25watts at 0.5ohms:
V*V/0.5 = 25
V*V = 25*0.5 = 12.5
V = sqrt(12.5) = 3.53
Amps = Volts/Ohms = 3.53/0.5 = 7.07
At 1.5 ohms, you're drawing a little over 4 amps, while at 0.5 you're drawing nearly twice that much at more than 7 amps. A 2000mAh battery, for example, will last a little less than 30 minutes (of actual firing time) at 1.5ohms, and just a little over half that long (less than 17 minutes) at 0.5.
A higher amp draw leads to shorter battery life, and lower ohms (or higher voltage, or higher power -- they're all related) means a higher amp draw, no two ways about it. "When a baby throws his rattle, all the stars in the heavens move to oblige." To keep the same amperage at lower ohms, you would have to reduce the voltage (and thus, the power) as well, to compensate. 2000mAh means "two thousand miliamp hours" which is equal to "two amp hours." That means it can output a current of two amps for an hour, or the equivalent (one amp for two hours, or half an amp for four hours, or four amps for half an hour, etc.) Any way you slice it, it comes out to two amp-hours. So if the amps go up, the hours go down. Simple as that.
Math'd!
[Edit: Retract'd! See rebuttal on pg3.]