Simitai closed for redevelopment a couple of weeks ago. Depending on which authority you refer to, this could take 2 or 3 years. However, this is China, so anything could happen, including abandonment of the plan and re-opening. If planning a trip to this area, check status and be prepared to go to Great Wall Plan B location. There are other areas of the non-reconstructed/undeveloped Wall available for viewing, should you have time and not want to go to the more visited parts at Badaling or Mutianyu. Some of these areas are not officially sanctioned for the public, many take 3 hours each way to access, and most require reasonably good level of fitness.