How long should 8, 1.5V batteries last for?
Do the math.  You say the thing you want to power takes 300 mA at 12 V.  You are powering it from a 12 V 2.3 Ah battery.

&nbsp;&nbsp;(2.3 Ah)/(300 mA) = 7.7 hours

That's of course the theoretical answer.  There are a number of things that conspire to make batteries much less predictable than a simple amp-hour rating.  The amp-hour rating the manufacturer quotes is usually under the most favorable conditions, so you'll likely get less.  At low temperature you can get a lot less.  You're probably fairly likely to get at least 6 hours run time at room temperature with fully charged batteries in good condition.