Mine has been charging for at least 9 hours and I've only managed to get it from 10 percent to 23 percent while using it. I have no idea, it tends to charge faster when not being used. Although its annoying.
IIRC, 4-6 hours for a 2, 8-10 for a 3 (depending on how far down it is of course)
You also need to charge plugged into the wall with the supplied charger. Trying to use an iPhone charger or via USB will work, but will take significantly longer since neither of those methods supply enough power to meed the iPad's minimum requirements.
Also, if you're using some sort of dock, it needs to be an iPad dock, not an iPhone one, or it likely won't put out enough power. and yes, it does charge faster if the screen is off. For example, if I have it plugged into my computer there is a trickle charge via USB, but if the screen is on, the iPad is using as much if not a bit more power than the USB can put out so the bucket never fills, so to speak.
If your device is plugged into the wall and is taking too long to charge, you may want to make an appointment at your local apple store and have them take a look at it. (Especially if it's still under warranty).
The quickest way (and really the only way) to charge your iPad is with the included 10W USB Power Adapter. iPad will also charge, although more slowly, when attached to a computer with a high-power USB port (many recent Mac computers) or with an iPhone Power Adapter (5W). When attached to a computer via a standard USB port (most PCs or older Mac computers) iPad will charge very slowly (but iPad indicates not charging). Make sure your computer is on while charging iPad via USB. If iPad is connected to a computer that’s turned off or is in sleep or standby mode, the iPad battery will continue to drain.
Apple recommends that once a month you let the iPad fully discharge & then recharge to 100%.
How to Calibrate Your Mac, iPhone, or iPad Battery
At this link http://www.tomshardware.com/reviews/galaxy-tab-android-tablet,3014-11.html , tests show that the iPad 2 battery (25 watt-hours) will charge to 90% in 3 hours 1 minute. It will charge to 100% in 4 hours 2 minutes. The new iPad has a larger capacity battery (42 watt-hours), so using the 10W charger will obviously take longer. If you are using your iPad while charging, it will take even longer. It's best to turn your new iPad OFF and charge over night. Also look at The iPad's charging challenge explained http://www.macworld.com/article/1150356/ipadcharging.html
Also, if you have a 3rd generation iPad, look at
Apple: iPad Battery Nothing to Get Charged Up About
Apple Explains New iPad's Continued Charging Beyond 100% Battery Level
New iPad Takes Much Longer to Charge Than iPad 2
Apple Batteries - iPad http://www.apple.com/batteries/ipad.html
Extend iPad Battery Life (Look at pjl123 comment)
New iPad Slow to Recharge, Barely Charges During Use
Tips About Charging for New iPad 3
Prolong battery lifespan for iPad / iPad 2 / iPad 3: charging tips
iPhone, iPod, Using the iPad Charger
Install and use Battery Doctor HD
In rare instances when using the Camera Connection Kit, you may notice that iPad does not charge after using the Camera Connection Kit. Disconnecting and reconnecting the iPad from the charger will resolve this issue.