In my experience, when the dye is gone, so are the conditioning benefits, at least to the point where it isn't worth the effort of putting mud on your head and then rinsing it out. The ROI at that point just isn't there.
Also, I did some testing TRYING to kill the dye in henna. I added boiling water and then microwaved it for several minutes, and let it sit out in a warm place for days.
After 52 hours it was STILL staining. It's very hard to gauge when you hit true demise, and for people with light hair, any lingering color could be disastrous.
I think the old advice of, if you want the conditioning without the red, stick to cassia, applies here.