What bus route do i take to go from Xian to Zhashui Karst Caves? how long is the bus ride?

You can go to local South Bus Station to take bus for Zhashui. Upon reaching Zhashui Bus Station, you can take local bus (Zhashui to Zhen'an) to reach the Karst Caves.
The bus for Zhashui operates from 06:00 to 19:00 and runs about an hour on the road. From Zhashui to the cave takes around half an hour.