Calculate battery life
I would like to know how to calculate battery life when using LED strip. The LED strip is powered by 8 AA batteries. LED Current Draw = 260 mA.


Right. The batteries will be discharged at 260 mA. This is close enough to the 250 mA of the third bar on the capacity chart so we can assume 2000 mAh capacity.


  This is my calculation, but I'm not sure if it's correct. Battery Life = (Battery Capacity)/(Load Current) = 2000 mAh /260 mA = 7.7 hours.


Correct.


  This is for one battery, and the lights are powered by 8 batteries, so is the total battery life = (7.7)x(8) = 61.6 hours?


You were doing fine up until this. Here's the way it works:


One cell gives 2000 mAh at 1.5 V.
Two cells in parallel gives 4000 mAh at 1.5 V. (But this is not your configuration.)
Two cells in series gives 2000 mAh at 3 V.
Eight cells in series gives 2000 mAh at 12 V.


Run time = 2000mAh / 260mA = 7.7 h.

We can cross-check this with the energy capacity which is given by V x mAh. 


Each cell has an energy capacity of 1.5 x 2000m = 3000 mWh.
Eight cells have a capacity of 8 x 3000 = 24000 mWh = 24 Wh.
Using your figure of 3.1 W the run time is 24 / 3.1 = 7.7 h.


The result is 7.7 h in both cases.