Over nine hours. That's how long the display was turned on while using the Xiaomi Mi Max I'm currently testing before the battery was nearly dead. The phone itself was off the charger for over 33 hours total.
That time saw mixed 3G mobile data usage and Wi-Fi, high and low brightness, Bluetooth on, all Google apps syncing, with no battery or power saving profiles active. I browsed the web, used social networks, took a few photos, watched Netflix and YouTube, and listened to music. Nothing out of the ordinary: this is just how the phone works.
The Xiaomi Mi Max has a 4850mAh non-removable battery and a 6.4" 1080p LCD display, which should make it clear just how such battery life is achievable. A lower resolution, an efficient chipset (Snapdragon 652), and a ginormous lithium-ion pack - plus Android 6.0's doze mode for idle time - are a recipe for longevity.
While it is understandable that a 1080p display may no longer be fashionable on a $700+ high-end smartphone today, what remains deeply mystifying to me is how most manufacturers seem to compromise on the size of batteries when battery life remains a massive pain point for most people, especially as their phones age. The fact that we may have access to chargers throughout the day or own external battery packs isn't an excuse: the entire reason we do these things is because smartphone batteries don't last long enough. The Xiaomi Mi Max, by the way, costs about $260 - so battery life really isn't about money.
Samsung, for example, has gotten a bit better this year, managing to cram a 3000mAh battery in the 5.1" Galaxy S7 - the Exynos version of the phone gets pretty great battery life, too - but hey, why not 3500? Why not 4000? Especially considering cycle life causes batteries to degrade over time and phones tend not to be used in a vacuum of imaginary battery benchmarks (people install crappy, battery-draining apps, that's reality), every extra bit of battery we can jam into a smartphone should be on the table. An extra two or three millimeters of thickness for an extra couple hours off the charger is an acceptable compromise until batteries themselves get better, or smartphones become an order of magnitude more efficient.
Sustainable battery life is a huge reason to hold on to a phone versus upgrade, too, particularly when batteries are no longer easily replaceable by the owner of the phone. Can you fit a 4850mAh battery into a 5" smartphone while keeping it reasonably thin? Probably not. But given that we managed 3500mAh in one three years ago with an 8.5mm cross-section, I think we could do just a bit better than we are now. Because when it comes to battery life, there really doesn't seem to be true a replacement for displacement just yet.