I'm looking to power a raspberry pi model 3 for a portable project. If I were to power it from a 10000mAh power bank (meant to charge smartphones), would the Pi be ok off this power supply, and if so, how long would it last on average? I am not looking to plug in any devices to the Pi, as my project solely requires software.
Try it and see! https://www.raspberrypi.org/documentation/hardware/raspberrypi/power/README.md suggests 400mA so (10,000 / 400) * (3.3 / 5) ~ 16 hours.
Unfortunately Power Banks differ widely in their ACTUAL performance.