That's over 12 hours, and sounds about right to me. I believe Fiio claim about 13 hours using Bluetooth, and a bit less using wired headphones. In normal use though, I find it won't get close to that, but i don't think the battery life is bad for such a small device.
Are you getting comparable battery life using wired headphones? I really don't think the M6 will last 13 hours playing music over bluetooth, as from my experience turning on Wifi and/or bluetooth drains the M6's battery at a fairly quick pace...