Ellie,
If I recall correctly, the iPad is like the iPhone in that it takes a full charge then cuts off the charger. It will not harm the battery. I do the same thing when I hit a low charge (usually every few days).
Most of the time, I just plug in occasionally to sync files. All though I have found on occasion I will leave it with an aquarium app running propped up next to my desk while I work. It goes to sleep after a while, but makes for a nice desk decoration when not in use.
Otherwise, the usage with occasional recharges ends up with a charge in the teens every four or five days. Then I will leave it charging overnight, top it off at 100%. Then repeat and rinse.