Single board computer draws 0.22 amps: How to calculate how long a 2400 mAH 3s Lipo will provide power?
2400 mAh / 220 mA = 10.9 Hours. Not including regulator inefficiency, changes in the battery over time, or peak draws that your multimeter cannot see.

But we have to adjust for the voltage differences.

11.1V * 2.4 Ah = 26.64 Wh available. Take 80% for typical worst case step down regulation, 26.64 Wh * 80% = 21.31 Wh. At 5V, that's 21.32 Wh / 5V = 4.26 Ah.

So 4260 mAh / 220 mA = 19.36 Hours. Again, round down a bit.

So probably more like 18 hours, give or take.

The reality will change depending on how much power is actually taken, or how often the RPI is asleep.