100% White Hair coloured Brown w H.L.'s

My hair is short, layered, white but have been colouring it brown for years. A few years ago got blonde highlights, which i love and now i don't like when the h.l.'s grow out and it's all brown. At the moment i have 6 weeks of white roots, lots of h.l.'s over brown and i want to go all blonde and then have just the roots done every 4 or 5 weeks. (h.l.'s too expensive to keep up). Are there any tips you can recommend that i can request of my hairdresser to avoid too much damage? Any idea how long this will take?