I have never measured it. At worst the main power has gone down for a 2-3 hours and it lasted at least as much. I don't have a USB power meter to measure the power taken by the Rpi so I can not give you an exact number but I would guess it to be at least around 5-10 hours if not more.

Edit: Someone linked pijuice project. I went to their website and according to them, Their 10k mah version lasts 24+ hours

> Onboard 1820 mAh off the shelf Lipo / LiIon battery for ~4 to 6 hours in constant use! (with support for larger Lipo Battery of 5000 or 10,000 mAH+ to last up to 24 hrs +)

https://uk.pi-supply.com/products/pijuice-standard

So, I would say 2i would probably last about the same?