Every henna is different, and ambient air temperature, humidity, as well as the recipient's body chemistry, can radically change this.
My henna (I am a henna artist, not hair dyer ) is ready when a small blob stains the inside of my wrist orange within five minutes. When the color is intense, somewhat neon pumpkin, I know the henna has reached max staining potential (I do not use the term dye release, since we do not really know what is chemically going on with henna).
Recently I got some Rajasthani henna that seems to need 3-4 days to be ready. I mix it with hot water (no acid, since we discovered that this causes lighter staining) and honey, then a day later add my essential oils. Let sit covered, test 3-5 days later, it's usually ready. It is often ready faster in hotter weather (which we rarely have where I live).