Time Necessary for a Tour to Beng Mealea & Koh Ker
It is not possible to do Beng Mealea, Banteay Srei and Koh Ker adequately in one day. However, Koh Ker is definitely not the one I would eliminate. In fact, it is the best of the three. It takes a couple of hours to get to Beng Mealea from Siem Reap. We did Beng Mealea, including the back and west sides in ~2 hours. It took ~1.5 hours to drive to Koh Ker from Beng Mealea. We spent ~2.5 hours there, stopping at several sites on the loop road, walking back to the giant pyramid, and having a quick lunch. I used the time stamps on my photos to estimate the times. Therefore, for a full tour of each site you are looking at a ~11.5 hour round trip from Siem Reap. I had hired a car and driver. A set tour may take a shorter path at each site and may take less overall time. The admission at Beng Mealea is 5 USD plus 2.50 USD each way for the private toll road. Koh Ker has a 10 USD entry fee, which you can pay onsite or at the Beng Mealea toll booth. The Mandalay Inn travel office offers a Koh Ker, Beng Mealea and Bakong day tour. They also have a Banteay Srei, Kabal Spean, Beng Mealea and Bakong day tour.