I've left phones plugged in for over a month (it happens in a shop if the phone gets shoved to the back of the bench and forgotten). I usually keep all the phones and Bluetooth headsets I'm not using on chargers until I need them.
Lithium charging circuitry (the circuit in the phone) turns the charge off when the battery is fully charged (which is about half an hour after it reaches "100%") - then, if the charge drops a little, the charger kicks in, brings it back to full charge and shuts off again.
Leaving it on the charger until you're ready to use it endures that you start with a fully charged battery.