If you've got virgin hair (i.e., hair that's never been dyed before), you might need to let the dye sit for a shorter amount of time, around 20-30 minutes. But, if you're a hair dye veteran, with locks that have been dyed, re-dyed, and re-re-dyed, you might need to let it sit for a bit longer, around 30-45 minutes.
Permanent Hair Dye Processing Time: How Long Should It Sit? | ShunSalon
And then there's the issue of porosity - yes, it's a thing! If your hair has low porosity, it might take longer for the dye to penetrate, while high porosity hair might soak it up like a sponge. It's like your hair is playing a game of dye-vasion, and you need to figure out the perfect timing to get the color just right.
