when does the bus from Jiuzhaigou arrive in Lanzhou?

The bus duration is hard to guaranteed, all depends on the traffic conditions. As i know this trip need about 10 hour by car and if you take the rest time, lunch time etc into consideration, you may arrive Lanzhou around 6 or 7pm. It very hard to say if you can catch the 7:30pm ride to Dunhuang. If the tickets are plenty, you may choose to buy it after your arrival. If limited left, you may think about buying a ticket for the next day and if you arrive at Lanzhou ealy, you can change the ticket to that 7:30pm ride.