I haven't redyed mine in quite a long while, I think actually early january is when I last hennaed it. The color is still gorgeous and bright, takes well on my blonde+light-brown hair. I have growth at my roots which is blonde-brown currently, but I'm kind of digging it. was thinking about doing another henna sesh very soon though so we will see what route I take with it.
I find that if you dye your brows they need it more often than your locks since they replenish themselves more often. I'm sure it is also dependent on your natural hair color underneath. Henna is very permanent though, and you can't do much good dyeing over it, so make sure you know it's what you want :)