I've had my t-mobile S6 for 1 week, this is my 2nd one!! I thought I had a lemon the first time so I exchanged it for another. Coming from an S5 the battery life on this S6 has been terrible, everything else on the phone I love, but the anxiety I feel as I watch my battery drop over 10% per hour in standby is ridiculous, if I use the phone it drops 15%+ per hour. I've done a factory reset 3 times. This last time I didn't even install any apps or turn on wifi, I went through and disabled all the bloatware, voLTE, and all the other tips people suggest. Even with no bloat apps, no location, no sync, no wifi, no google account, & fresh reset it still drains 10% per hour in standby.
Not sure what else I should do, if I can't figure it out I guess I'll go back to my S5 which would last me easily from 8 am to 10pm. I'm not a heavy user at all, 1-2 hour SOT is probably all I need plus 10-12 hours standby. Never been an issue with any of my Android phones dating back to the G1.