Leshan to Chengdu

Hi. I plan on spending one night in Leshan (coming back from a Emeishan trek), see the Giant Buddha in the morning and be back in CD for my 5:50 PM train to Xi'an. How long should I stay in the Buddha park and at what time should I take the bus to CD in order to be on time (an even a little bit in advance if possible) to take my train?
Thanks!