The road for part of the way has been upgraded as of earlier this year and new bridges/tunnels are still being completed. Current reports are Chengdu-Songpan (possibly worth a stopover): 6 hours, and Chengdu-Jiuzhaigou: 8 hours. I would allow an extra cushion of time however, if you are trying to connect up with a night flight the same day. Get the earliest bus you can back to Chengdu. You'll need to allow time to get from bus station to airport, and for check-in. In other words, get back to Chengdu city by around 19:00. If flights are halfway reasonable, I'd probably fly J-->C