Topic: How often can you apply henna?

I just used the dark brown henna for the first time on my hair and am very pleased with the results. How often can I apply the henna to my hair? I've used other brands in the past that have faded pretty quickly. The color is a deep dark brown that I'd like to maintain for as long as possible. Also, I'm getting some dark greenish residue on my pillow case and on my fingers when I run them through my hair. Is this normal for fresh henna? Will it go away or did I not apply it properly?