You need to measure the actual tail current draw or estimate it. Then knowing the capacity of your 18650 cells, you can calculate the run time.
To estimate the tail current draw, you need to have an idea of the actual LED forward current (let's say it's 2.8A), the forward voltage (let's say 3.5V), battery voltage (7.4V), and driver efficiency (85%). Then power delivered by the driver equals power drawn by the LED:
driver efficiency * Vbatt * Ibatt = Vf * If
0.85 * 7.4 * Ibatt = 3.5 * 2.8
Solving for Ibatt, we get Ibatt = 1.558A (you'll get the same # using the online runtime calculator). Dividing Ibatt into the cell capacity (let's say 2500mAh) gives estimated run time:
2500mAh/1558mA = 1.6h (or 1h36m, which is what the online run time calculator gives)