From dark to blonde..how to?

Hi,

Since I was 11 I have been using store bought colourants on my hair (I realise at now as an adult how bad an idea this actually was!) and I have had my hair black now for several years, however, my natural colour is blonde and I want to go back to that, how do I do this, obviously it will be gradual process, but my main question is just how many trips to the salon will this take and how much is it likely to cost overall? Any tips or advice more than welcome! Thanks in advance!