Hair color question: how often will I need to go back to the salon to touch-up my roots?

I just had permanent hair color done professionally on my naturally dark brown hair. The new color is fairly close to my natural color, only slightly lighter with some reddish tones (she did a medium golden brown, but my hair pulled in it`s own red - I don`t mind though, since it`s not toooo red).

I colored it mainly to cover my grey (I am about 20-30% grey) and to give my hair a touch more highlight and pizzazz. How often will I have to touch up roots? My hairdresser says I should only have to go back every 8 weeks, but that seems like almost too good to be true. Is that reasonable or will I need to go back more frequently?

Thanks!!!!

well, it just depends on how fast your hair grows. for average or slightly slower growing hair, i`d say this is about right. if your hair is average to fast growing ie- have to get it cut every 4 weeks. then you`d probably hafta get it done every 5-6 wks.