In my case, with long hair, it took three years to completely grow out and, along with menopause and related weight gain, it has only served to make me feel old and frumpy. So I decided to have it dyed again, but the dye wouldn't ‘take' on the fully grey hair! It only showed up on the underlayer where some of my original natural colour remained.
Not sure why it worked when I was only having roots done, but won't stick now...maybe because the rest of it it had been coloured over and over which had changed the texture.
Sorry to be the voice of doom, I do hope everyone else enjoys their grey hair and has better luck.