LOL Yay, it's a grassy smell and I've noticed it plays better with some product smells than others...(Henna+the citrusy Oyin Honey Hemp=) I'd say it takes a week, but it probably depends on how often you cleanse your hair. Can I asked what you mixed your henna with? Some liquids, like red wine, can contribute to the problem. I've heard adding powdered ginger or cardamom to the mix can help.
Glad to hear you lie the color!