Charging time of battery varies based on which phone, what initial charge is, what else phone is doing, and what charger you use.
4, 4s, 5, 5s, 6 all can charge battery at max rate of about 6 watts. So using the charger they came with, initial residual charge less than 50%, screen off, radios off like airplane mode or phone turned off completely. You will get all wattage going to charge the battery. As the phone charge increases, the phone will regulate how much wattage it uses to charge the battery till it is at trickle charge at 99-100%
If you just have it in sleep, the radios will still send and receive using some of the power, so longer charge time. If screen is also on, even more power diverted away from charging battery. And if you are actively using the phone, charge time will be longest.
If you use the iPad charger on these phones, there is enough wattage to send maximum 6 watts to charge the battery and still have enough for the radios, screen, and even fully run the phone. That is why people report faster charge times using iPad charger with these phones. They will not see faster charge if they turn off all other functions, screen radios etc.
The 6+ acts the same way using the charger it came with. However, it is designed to accept the full 12 watts from the iPad charger for charging the battery, when none of the power is used for radios, screen, running. If you have screen on, if radios work, or are using the phone then some power will be diverted from charging, so longer charge time for battery. Just like all iPhones, the 6+ will regulate the wattage going to charge the battery as the battery fills. When it reaches 99-100% only a trickle charge will be used regardless of what charger is connected.
You can't damage the phone or the battery using the Apple chargers and cables.
The fastest charge of the 6+ will be using the iPad charger with the screen off, radios off, ie airplane mode or turned off completely. Try it yourself, take 6+ at 20% hook up iPad charger and put phone in airplane mode, turn off screen. Come back in 20 minutes, check battery level. It's like super fast charge some other phones say they have.