I've shifted the use of my henna further and further apart over time. Since I've been MT'ing, I'm using henna about every 3 months, now, and that works for me. Henna acts like a protein on my hair, and I didn't want to overload, so I waited til my hair 'told' me it needed henna (loud and clear, too), and it was about 3.5/4 months after my last one, so every 3 months is about right.