Question

How long should I fully charge a battery before turning on my smartphone?

Answer 1

It shouldn't make too much of a difference, It may charge slightly slower if the screen is on, but if the phone is completely flat I would say give it 5 minutes first so it has enough power to go through its start up process.

Answer 2

If it's a new phone, 24 hours it's a good time to let it fully function correctly.

Answer 3

Up to you. Anyway or anyhow it is ok.  Either you wait till it become full charged or open it as soon as you plug the charger.