i arrive at 12.15 by plane to chengdu and we are thinking of going to mt Qingcheng,

Well, to be honest, the time is a little bit tight.
Upon landing, take a long-distance bus at the airport, and get off at Dujiangyan first. The schedules are 11:20,12:30, 13:30, 14:20, 15:10, 16:10, 17:10 and 18:00; and duration is about an hour. Then take a taxi to the mountain directly. After arriving, you need to tour around the scenic area as soon as possible.

BTW, there are bullet trains from the airport to Emeishan directly with a duration of about an hour. Still, time is too tight to climb the mountain.

And which are the hours to come back to chengdu?

From Dujiangyan come back to the airport, the schedules are 09:45, 10:35, 11:25, 12:20, 13:30, 14:30, 15:40 and 17:10.

Actually, from Qingchengshan, you can take a bullet train back to CD railway station in downtown directly; in the case, you can avoid the transfer from airport to downtown. The schedules from Qingchengshan are 17:42, 21:54 and 22:18. Duration is about 40-60 minutes and ticket fare is only CNY15.