Does semi-permanent hair dye wash all the way out eventually?

Does semi-permanent dye wash all the way out eventually? I'm thinking of dying my hair (I currently have light brown hair that's a really drab color and want to a darker maybe ashy brown color) but I want something low maintenance and that won't leave bad root lines as it grows out...so will it wash out or fade out and blend as it grows out, or am I just going to have to keep touchig it up? Thanks!