I just did a test to see how long the battery in my 8 would last without recharging. About 40 hours to get down to 15% at which point battery saver mode started. I used my phone normally (for me) during the time, which means web, emails and news feeds, little video or audio, and low screen brightness. I've previously followed the steps to extend battery life listed on nokiapoweruser website, which i found helpful.
How long can your phone last?