The battery life was an area where I was pleasantly surprised. A 1400 mAh unit is not big by modern standards, but then again the First 6's display is low-resolution so it requires less power than an HD panel, for example.
In real world terms, I found the battery life would comfortably last a day and a half, perhaps even two days with less regular use. This is with mobile data always enabled and auto-sync on. My reasoning for this impressive result is that neither the display or the CPU seem very power-hungry, so the first 6 can soldier on longer than some more expensive phones.
A benchmark time of 9:07 confirms this, with the first 6 becoming the longest-lasting phone yet in the Geekbench 3 battery benchmark.