If there is nothing worse than having your phone's battery die, it would seem that there is no harm in leaving your phone plugged in for longer than it takes to charge to 100%. But this is actually a huge misconception. Charging your phone all day can in reality damage the device.
A lot of people believe it is a myth that over-charging your phone can damage the battery, but research has proven this to be fact. Leaving your phone plugged in when it is fully charged can harm the battery in its overall lifespan.
Once a phone's battery reaches 100% charge, the battery receives it small "trickle" charges to maintain full charge if it is still plugged in. This keeps the battery in a state of high stress wearing down the chemistry inside. Meaning you might not see a damaging effect on your phone from overcharging it once or twice, but if you make it a habit your phone battery will weaken over time.
Instead of charging your phone all day or forgetting to charge it until the battery dies, find a happy middle ground. Plug your phone in when it reaches a low percentage, then keep an eye on it to see how long it takes to achieve full charge. Then, you can set a timer for yourself every time you charge the phone so you'll know to unplug it once it reaches 100%.