My Phone is a unlocked Lumia 1520, under WP 8.1, I could go 36-40 hours on a full charge. When I ran the upgrade, my battery was fully charged, and in less than 1 hour, the upgrade process completely drained the battery to "Zero". Curious to hear about others with they 1520, and your experience.
For me, I kept the phone plugged in the entire time, and the upgrade took a very long time to finalize everything once it rebooted. The latest build ran smooth, and I didn't notice any lags in the screen scrolling, or switching between Apps. However, the Phone ran very HOT! Especially around the Camera area of the body. Even after rebooting several times, the phone continued to heat up. I had read of other stories of phones "Bricking" as a result of excessive heat, so opt'd out, and rolled back to WP 8.1. Just my experience.