What to do about dark roots??

My hair is naturally some kind of brownish dark blonde and I had mine dyed platinum. I did my roots every 3 weeks and no one ever really noticed them. I knew a girl with black hair who went platinum and she had to have hers done every 2 weeks (this is just to be sure no one ever saw them, if you didn't worry about it and let it go a little, you could wait longer).

I have dark brown hair and would like to do more of a all over honey blonde highlight look. The only thing that is holding me back is the thought of dark roots! Is there a way (at the salon) that the stylist can just touch up your roots every few weeks? How often do you need to get your roots done? Thank you!