I've got dark hair and when I was blonde I was re-bleaching the roots every 1-2 weeks (3 weeks was too obvious). I was lucky that one of my girlfriends was a hairdresser so was doing it for me on the weekends. I had to start off with short hair because I needed to get rid of any hair that had previously had colour. Then we bleached it (from memory 3 or 4 goes to get to the blonde I wanted), treated it (I think it was something like a primer), had regular moisturising treatments. I also used blonde shampoo & conditioner.

Now if I'm going a drastic colour change, I have it on the underneath layers of my hair, currently I have the top layer dark brown and underneath is a red colour (so you can't see the regrowth)