i heard that if you let the battery drain to zero that it could hurt the battery life (like if it goes to zero the battery life would go down from like maybe 10 hours, to 8 hours. just an example.) is this true because ive been worried about my batteries

It is not good for Lithium chemistry batteries to become fully discharged, however, iPads, iPhones and iPods will shut off before the battery is fully drained. That is, when the phone goes from 1% to 0% and shuts off there is still plenty of charge on the battery. However, you should not let it sit for a long time after it shuts off, as the phone still uses some power even in this state. So if the phone shuts off due to low power you should charge it within a day or two.

You also cannot overcharge an iDevice. The charger is actually in the device; it is not the adapter that you plug into the wall. That is just a 5 volt power source. And the charger in the device will stop charging when the device reaches full charge.

One advantage of Lithium batteries is that they have no "memory effect". That is, you don't have to fully drain them before charging, and you can connect them to a charger at whatever the current state of charge, and remove them from the charger if necessary before they reach full charge. None of these activities will shorten the life of the battery.

It is a good idea to run the battery all the way down, then charge to 100% every month or so. But this is to recalibrate the battery gauge, it is not to "condition" the battery.

I don't know for sure, because I've never let it go any length of time. I shut connect it as soon as it shuts off because I don't want to be without my phone. I know a fully charged phone that is turned off will keep a charge for several weeks; my spare 4S does.