Any time you go from a darker to a light base, there will be a significant amount of contrast at your roots. Normally clients of mine that have similar coloring come in every 4 weeks for a solid color or 4-6 wks for highlights. Either way the maintenance is something you will have to do more frequently. I would suggest going for a full head highlights. This will lighter the overall appearance of your color while keeping some of your natural intertwined to serve as lowlights. This will really help to extend the life of your color before needing to get it touched up.