You should only be using the charging unit that came with the iPad, it will say 10w instead of 5w like the iPhone.
As for 15 hours...not a lie at all. I used mine yesterday moring for an hour in the morning, I worked from 9am until 8pm yesterday, on all day, I had 20% left when I left. I went home used another 2 hours and had 7% left when I went to bed. That's 14hrs on with a little over an hour left. I leave the screen on all day when I'm not using it because it lasts all day.
Yeah with that math i guess it is 15 hours, so at your job you were on it the whole time or most of it. Anyway if this is true then I'm glad because apple will be replacing my ipad. And I am using the charger that came with it. But for one the brightness was my main issue it really kills the battery. Try urs on full all day and tell me how long it last, spend about half of that time gaming and streaming music.