This has been my biggest disappointment of my Note 5 so far.
I have had this phone for two months now. And the battery life is abysmal. At first I wasn't able to get 18 hours standby with barely 2 hour SOT. There's an issue with WiFi, and you have to disable "Keep Wi-Fi ON during sleep" helped a lot..
After doing that, getting rid of bloatware via Package Disabler Pro, I am able to get through 24 hours, with 4 hours SOT. With "restricted background data" I might get through 36 hours.
WiFi seems, to me, to be the biggest issue.
Here's a screenshot:
I will however admit that my screen brightness is set to 65-70%, non-auto.
If I had an option, I would have returned this phone, and gone with the 6S+.
This is a SM-920I.
I'd also look at the XDA thread about battery life. Someone people are doing much better than me.