We have some hairdressers on the forum who are best equipped to answer - I can only tell you my own personal experience!
I can go a lot longer between highlights if the highlights are only 1, at most 2, shades lighter than my natural color. I.e., very subtle highlights. The roots are just not as noticeable as they grow out, and the hair color fades.
The past 6 months or so, my hairdresser went maybe 3-4 shades lighter? It seemed so to me. The roots were much more noticeable - beyond my preference. But one trick I have learned from hairdressers is to part your hair assymetrically (a zig-zag part) - this makes the roots not so noticeable.
With that said, I can still only go about 8 weeks, absolute tops! Even if the roots are subtle, my hair just simply goes flat at the crown, without the benefit of some dye. Hair coloring changes the texture of my hair in a positive way, and even if I was to stick to my natural mousy shade, I'd probably still get it colored, just for the texture and shine boost.
To eek out 8 weeks, I use shampoo designed for gray hair. Try Aveda. It is purple. It combats brassiness. It's a bit drying so only use it once or twice a week, not every day.
My VFGF Jane has her hair highlighted (in something like 6 shades of lowlights and subtle highlights). She has it done only twice a year. Her hair is long. It grows out gracefully, but then again, she's stuck to shades that are close to her natural color. Also, she only shampoos once a week! I'm always amazed when people can go that long between shampoos. I can't. But with hair color, the less you shampoo, the better.
