TL;DR: 3-5 hours at moderate load.
They use the Ah rating for the li-ion batteries at nominal 3.7V. You can always stick semi-expensive panasonic 2800mAh 18650s in there if the vendor has lied about the capacity. I use some from a golfcart in my USB batteries. Gung-ho.
So you have 3.7V*2.4Ah=8.8Wh. Assuming a boost efficiency of 85%, you have 7.5Wh.
In my many tests, I’ve had Pi1 and 2 far below 2.5W, usually peaking at 1.5W (I don’t overclock). 7.5/1.5=5 hours. EXPECT less, maybe 3 hours.
Can’t recommend the USB ampmeters, use a DMM set to ‘A’, and measure current in series with a pair of clip-on probes.
For power longevity, you can do a few more or less invasive things, like cut off LEDs, use memory filesystems, and use google to find powersaving tricks for Pi. I’ve seen libraries for a greener Pi experience.
It is better, however, to use a car battery instead, with a quality step down converter to 5.1V. Stepping down voltage is always more efficient than boosting it. Use a LVCO of 11.6V, you’d need e.g. a TLV431 with vref 1.25 for this... https://electronics.stackexchange.com/a/318196/147045
BTW: The TLV431 can be used with Li-Ion batteries - like venerable 18650s - a recommended LVCO is 3.3V for a parallel bank, or 6.6V for a parallel-series bank.
A nice 90Wh S/LA battery is cheap and small. About the size of a baby’s head. A big battery of about 720Wh is the size of lady ada’s head (but not pink or purple), and would let you run at MAXIMUM POWER, SCOTTY! (12W I assume) for <=60 hours...