How many hours should I charge iPhone 6 plus for the first time?

unplug it when it reach 100%. The first time isn't different from any other for a lithium based battery

I would recommend at least 4 hours or until it is showing 100% on the battery meter. Once the phone is fully charged the electronics will take over and just allow a trickle charge to keep the phone fully charged without hurting the battery.

Tushitushi wrote:

How many hours should I charge iPhone 6 plus for the first time?

The phone will come with some charge. You can use it right away. There is no need to charge it first. You can charge lithium ion batteries whenever you need to or whenever is convenient. You don't have to wait until they drain to a certain point and you don't have to charge them until any particular point.