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
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.
I was referring to the "Power Pack" usage which is what has been trying to charge the unit for well over 16 hours but still has not brought it up to 100% in all that time. I have just done some work on the iPad and now the power level is down to 57% which seems to indicate to me that the Charger Power Pack may not be working properly.
Incidentally, I am a retired RadioMan/Telecommunications Specialist so am quite familiar with proper battery usage and charge cycles as that was standard procedure with many Emergency Radio units we were require to keep charged and ready for use.
Sparkgapper=An ANCIENT Radioman
Most of that boilerplate comment above is out-of-date and doesn't apply to the new lithium-ion polymer batteries that Apple uses in iOS devices today.
This is what Apple says about its batteries;
Some folks report that to get the battery on the iPad back to correctly functioning, go to Settings/General/Reset and choose Erase All Content & Settings. Recharge the iPad with the AC wall unit and then reconfigure the iPad as a new device.