I use my Nexus at the gym for music...it drains almost NOTHING running MP3s and bluetooth (I use bluetooth earbuds). It is actually kinda weird, because it feels like it should be more. But I lose about 3-5% per hour. At that rate, I could run MP3s for well over 24 hours straight.
It seems weird because bluetooth should be making it drain fast by itself. The display is off most of the time, but music runs continuously, and I am also checking emails/text/whatever between sets, and sometimes web browsing.
That is because the Nexus 5 (with 4.4.4) was designed to give good battery life while playing music
From Android - 4.4 KitKat
"Low-power audio playback
Android 4.4 on Nexus 5 lets you listen to music for longer - up to 60 hours of audio playback.*
*Currently only available on Nexus 5"
