I have only had one time-extended dream while lucid, but it was incredible. I was dreaming for maybe 4 hours, and I experienced two days while in the dream. Having the power to fly for 48 hours made that my favourite dream ever. I flew to a mountain range, over jungles, across oceans, and hung around islands for hours. What really freaks me out is how fast this dream must have been happening in real time. I suppose dream-speed is restricted only to the speed of thought, which is amazing.