Hi Dost Log,
Though I could not go to Phong Nha caves due to ill health, I did all the research required to go there from Da Nang. There are direct buses that ply from Danang to Phong Nha. If you take one at 01:00 PM, expect to reach at 07:00 PM. You could also take a train to Dong Hoi from Da Nang (max 4.5 hours), then get a minibus or private taxi to Phong Nha (1 hour). There is a train that departs from Da Nang at 06:40 PM and reaches Dong Hoi at 01:00 AM. Your best bet then would be to spend the night in Dong Hoi and head to Phong Nha the next morning. Regarding the bus timings, it is ideal that you contact your hotel staff in Phong Nha/Da Nang or find out details at Da Nang Bus Station. Hope this helps. Happy travelling!