Re: Dawn will be orbiting Ceres for a very long time
The mapping orbit allows the spacecraft to photograph Ceres at lowish resolution, and in doing so allows the construction of a broad-brush map. By lowering the orbit scientists can start to look at interesting pictures identified from the map at a much higher resolution.
Oh yes, Dawn uses an ion engine; it has a low thrust but does not use much in the way of consumables. At the moment Dawn has plenty of fuel; don't forget this was all accounted for in the overall mission plan.