I re-henna my hair every week, because I think a lot of the problem with gray popping up so quickly is re-growth, not dye-failure. Grays are very eye-catching, especially on darker-haired folks because the contrast is so great, so it always looks like more than it is. Doing weekly roots-only hennas has solved my problem. Sure, it's a pain to color every week, but I figure if I'm going to dye and keep up the pretense that I'm not gray, then I should do it often. I hate seeing women walking around with gray roots showing, and I hate it even more on myself.