How long does bleach take to wear off?

Ok so I really want to dye strands of my hair blue with Raw true blue dye. My hair is a dark blonde and I only need to lighten it a few shades. As far as bleaching it first goes, I need to know how long it will take before my natural hair comes back where the bleach is? I heard that it never comes out because it destroys the color pigment in the hair, but how long till it goes back to normal?