How long b4 henna actually darkens hair

I'm wondering how long or how many applications might it take till the hair is made a dark color? I'm wondering because of Nikki's hair and how in earlier pics her hair is a brownish color but now pretty much black. Does anyone have an idea on this? I'm asking cause I have naturally black hair but half of it is grown out medium brown color. And this part is especially reddish after I do henna. I have only done henna followed by an indigo treatment so I don't even know how dark the henna will get or settle to if I let it oxidize over a couple of days instead of doing indigo after. Thinking bout trying the henna w/o the 2 step just to see though...