How long should it take an iPad to charge?

I was just issued an iPad for work. It was completely dead when I got it so I plugged it in at about 7:30pm. It's currently only 9% charged. It came with a 12 watt charging adapter... It is not a new iPad though, it's been used by past employees. I know it should take hours to fully charge from 0%, but shouldn't it be more than 9% having been plugged in and the screen in the off/sleep mode for 2.5 hrs??