I only charge when it's at 20 % when charged I unplug... The battery last longer when it's not overchared
Actually consistently running down lithium-based batteries, such as in the iPad, will shorten their life, not lengthen.
It's best to top if off when possible. But it is also good to use it regularly too. So if you don't use your iPad on battery power too often it is recommended to go through at least one charge cycle per month. Note that charge cycle does not have to be done at one time: if you charge it when at 80% then doing that 5 times would count as only one charging cycle.
Finally, there is no such thing as overcharging these batteries. The charging system is very well matched to the battery and will not overcharge it. If it did we would have iPads burning up left and right as overcharging lion/lipo batteries can be risky and dangerous. It quick-charges to about 80% then trickle-charges to 100%.
Michael