Radiant fire crystals to heat an item: 453
If you use the 100% option and get no double level ups.

Number of radiant fire crystals I get from Dreams and Nightmares: 23
This is an average; I collected data when the mission came out

Time it takes an above average party to complete Dreams and Nightmares: 15-30 minutes
15 with the absolute best players in the game I could find using the fastest strategy; hardly reasonable.
25-30 is generally what happens if I join random players with a decent loadout.
I've timed this (informally, didn't record it anywhere) for my own purposes

Number of radiant fire crystals per hour: 46 - 92

Number of hours it takes the average player to heat something: 9.8 hours per item
And this assumes they're capable of running dreams and nightmares.. which is a big assumption.

Number of hours to heat a loadout of 5 items: 49 hours of D&N
You see them complaining about finding 4* orbs? Yea. The frustration is delicious.

Number of hours it takes the best of the best to heat something: 4.92 hours per item
When something is unfair but unfair in your favor.....

Minutes it takes to load Dreams and Nightmares/watch cutscenes: ~5
Yes, I did time this as well.

Time spent watching loading screens and cutscenes per item: 1.64 hours of nothing but cutscene
And that's per item.