Hi Melida,
CLASS observations for Prek/VPK consists of 4 cycles of 20 minutes. After cycle 1, the observer will take 10 minutes to score in or outside of your classroom then will resume with cycle 2 and so forth until all of the cycles have been observed. For each cycle, the observer will be looking for you to incorporate the Dimensions that are mentioned above. The more you incorporate the behavior markers of the Dimensions, the higher your score will be. You do not have to incorporate all behavior markers for concept development, but the more "how" and "why" questions, problem solving, allowing the students to be creative, integration and connections to the children's lives, that you use when interacting with the students; will boost your score up for the Concept Development Dimension. The observation is about 2 hours. For the time that the observer is in your classroom observing you; I suggest that you incorporate as many behavior markers as you can. This includes when you are leading small groups; the observer will observe you as you teach your students that are in your group and the same goes for centers. During centers, you will need to interact and ask as many open ended questions as you can in order to began a back and forth conversation. I hope I have answered your question.