I'm currently testing my RPi Zero W with temp, humidity and barometric pressure sensors hooked up. I have two 3,000 mAh batteries wired in parallel. They also go through an Adafruit regulator. Once I had my python script running, I disconnected the keyboard and mouse since they won't be connected when my final project is finished.
In this configuration, i was able to get 8 Hrs and ten minutes on my RPi 3B in the exact same configuration. I'll know today how long the Zero W will run and I'll post it here.
In the configuration mentioned, it ran just a hair, (5 minutes) over 14 hours. All it does is read the sensors once every 10 minutes and insert them into a MySQL table. I'm also running Lighttpd web service. Hope this is helpful.
If you decide to go with Lithiium/ion batteries, be careful; there is some real garbage on ebay and amazon claiming to be 9800 mAh. They're garbage. These are an honest 3000 mAh. Good luck.