Is there a bus from Nanjing to Beijing? And if there is what is the duration?

As I know, there is no direct long-distance buses between these two places.
Thus you are advised to take bullet train from South Railway Station and get to Beijing South Railway Station. The duration is 4-5 hours. The ticket fare of second class seat is CNY443.5, first class seat is CNY748.5 and business class seat is CNY1403.5.