How long is it take to go from Jiankou to Mutianyu

About 5 hours, depending one's phisical strengh.

I just went and followed these directions. There was not a bus to Jiankou, but you can find those really annoying dudes who want you to get in their car and pay them a bunch of money to taxi you out there. If you're trying to save money, try to find a hostel or something goes out there. Pretty sure a "bus" doesn't exist; I even asked someone from the tourist info center (北京旅行） and she told me that Jiankou is illegal, etc. And seemed certain there was no public bus. Don't rely on this information.