About two hours from Tianqiao to Shihua cave. If you take the direct tourism bus from Arrow Tower East, it is may be a bit shorter in time. The addmission fee is 70 yuan. Suggest you bring some food in your bag. Bus 917 will depart before 6am in the morning. I think about 7a/8am is okay to the arrow tower for the tourism bus. There is fixed time to come back if you take the weekend/holiday tourism bus. Ask the driver or ticket seller in advance about the time to come back.

Thanks so much Mr Qi!