I first noticed the difference July 31, sometime around 2:30pm after an update had clearly gone through. With Battery Saver mode on, my phone could last a few hours with the screen brightness down. Battery Saver would turn the phone's screen off if the iPhone was turned upside down or put into a pocket.
But on Sunday evening, my iPhone 6S's battery dropped what seemed like 10% in only 20 minutes when I was riding the New York City subway. I had to shut the app off to conserve battery life.
To test how the app sapped my phone's juice, I took Pokémon Go for a stroll on the subway again Monday afternoon. I kept running Spotify, too, lest I be forced to listen to other New Yorkers **shudder**.
In other words, I used my iPhone very much how a normal person uses an iPhone.