how long does it take to fix a broken or cracked screen for iPhone6

It all depends if what cracked the screen caused other damage besides just the damage to the screen. The exact time for repair can only be determined after Apple has had a chance to examine your iPhone.

You are aware that anyone who allows you to do that to their iPhone is forfeiting all future service of any kind from Apple with that iPhone?

You, of course, explain that to them before do anything to their iPhones don't you?

Yes. I realize that. I only repair iPhones for people who understand that and phones that are out of warranty. 99% of the iPhones with new screens I sell on eBay. (Mostly iPhone 4S's and earlier right now.)

I do it all the time and for me, and it takes about an hour to do it carefully. Others may be faster or slower, though.

In response to the original question, I would call Apple support or go to an Apple store and pay to have it done by them if it's still under warranty. They will probably replace the whole phone.