Henna is pretty much totally permanent. Totally! Permanent! Indigo fades on some people, but the most common scenario seems to be that it fades after the first application or two, then start to be permanent.
Don't gamble on indigo fading for you though. It's permanent for many after the first application, and tends not to bleach out ever. If you try to it becomes yellow bleached hair + blue indigo = green.
Indigo should only be used in conjunction with henna in some way. Otherwise blue + your greys = blue.