To be sure no one can tell, get a natural color. Don't go from being a sandy brown to a black. Since guys usually don't have an obvious part in their hair, its easier to hide when the roots show up. When I dye my hair, I usually get it re-done every two months for a medium-long hair style, or every month for a short hair style.
That's also because I've gone from brown to white blonde to black. When you do an unnatural color to you, you need to touch it up once a month. Also, be very specific when telling your stylist what color you want. It helps to have a picture. "Brown" is subjective.