I listen pretty much at the same volume as you with same headphones but i get easily over 14 hours.Is good to know that even when you are not using it for a long period of time, you should charge the battery. Maybe the previous owner didn't really charge it properly, or didn't cared too much about it.
I wouldn't worry if i were you,and anyway Charles will be very helpful when the battery replacing times come.
I wouldn't worry if i were you,and anyway Charles will be very helpful when the battery replacing times come.