Well, it has been a year since my first henna and I am still going strong.
I would never, ever go back to commercial color.
What I have learned is that I don't need to henna the length at ALL, it
never fades. It is nice to do a "henna gloss" every now and then to keep it nice and bright - but I have only done that maybe 3x.
A "gloss" is simply a blend of equal parts prepared henna and hair conditioner.
You glop it on, press into the hair and put on a shower cap or plastic wrap for an hour or two (I did 3 hours last time). I did this in tandem with my regular root touch up (which is easy because you don't need to be precise with henna, just squish it into your roots and "moosh" around and you are done).
The photo was taken in November - my daughter and I are having a kind of growing contest to see who can get the longest. Considering I am now past 55 I think it is kind of fun to have long hair - and the henna keeps it in optimum condition so it looks great.