how to get Longqing Gorge by bus from beijing prime hotel ?? how long does it take ??

From your hotel, you can take a taxi to Deshengmen first. Then take bus no.919 to Yanqing County first. Then transfer to bus no.875 to arrive at the scenic area directly. The whole way takes around 3 hours.
If you wanna take a taxi from the hotel to the destination, it would be CNY 300-350 for the single trip.
Besides, you can contact TravelChinaGuide and see whether if they can arrange such tour for you. As I know, they have reliable tour guides and reasonable prices. If interested, their contact information is traveltravelchinaguide.com.