How many hours would you recommend staying at the Great Wall and how long is it from Beijing?

Around three hours are suggested to spend there.
Badaling section is about 80km from the city downtown area and you can take the bus 919 from Deshengmen Bus Station to reach your there directly.
Mutianyu section is also about 80km from the city and you can go to Dongzhimen Bus Station to take bus 867 to reach there directly.