Calculating discharge time of 12V LED light powered by AA batteries
You want to get constant current at 12V to run the LED's from 1.5V batteries.

You will need a boost converter, as others have suggested, to raise the 1.5V to 12V. Power conversions are not ideal, and all voltage-changing devices consume/waste some of the input voltage doing their job. The constant-current chart in the data-sheet is meaningless since a boost converter draws a variable amount of current at a variable frequency and/or duty-cycle.

At the LED's, what you are asking for is:

Constant voltage(E) * constant current(I) = constant power (P)


Regardless of the conversion method, you will require the same input power from the battery as you want the LED's to output. Boost converters are very efficient, but do have associated losses. 

Pbattery = PboostLoss + Pled


and

P = I * E


So, for a boost converter to draw a constant P from the battery, as E decreases, I must increase, discharging the battery even faster, causing its voltage to drop faster, etc., etc. That is the reason there is not an easy or definitive answer as to how long they will last for your application. LTSpice is helpful for time-integrating things like this but I'm not sure if you can model a AA battery's discharge properly.

There are only two "absolutes" I can think of for this question:


In identical circuits, twice as many batteries in parallel will run for twice as long. Batteries discharge constant power, together.
Twice as many batteries in series will require less intense and/or frequent boosting to reach 12V, reducing switching losses. Batteries discharge constant power, together.


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A boost converter: https://www.pololu.com/product/799

Comparison: https://www.pololu.com/search/compare/132

When you go looking for similar trinkets, check SparkFun, Adafruit, and Pololu first!