With sitting poses, and especially with long full dresses, it's easy to have faces bunch up and intersect with other faces. Once things start to bunch up, calculations become slow if not impossible. For starters, you might try to check the "Cloth self collision" setting when you set up the simulation. If your final result is a still image, an alternative is to begin the simulation with the chair moved away from the dress, and the figure in its default position. Pose the figure and the chair in their final pose and position in a later frame, making sure that you have enough frames in your simulation to cover the entire range. As the chair moves toward the figure and into its final position, it will push, or displace the skirt forward toward the legs, much as the arms do when a woman sits down in a skirt. If you're doing an animation, the arms could control the skirt much as they would "in real life." Does that make sense?