Henna cannot lighten hair, so using a lighter color henna on dark brown hair won't change the color. Using a red henna can give red tones, but the lighter browns cannot do that.
Minimum time for the next application is 72 hours. However, it really depends on how fast your hair grows. The color doesn't really fade like a traditional chemical dye would. I personally wait about 2 months in between applications. -Hilary