5 hours and 55 minutes.

According to inquiries with Eastern Airlines, we learned that flying from Hefei to Nan Nha Trang requires three transfers before arriving. Take Eastern Airlines flight MU5870 to Kunming, and the flight time is 2 hours and 40 minutes. Take Eastern Airlines MU9605 to Hanoi, and the flight time is 1 hour and 25 minutes.

Taking Bamboo Airways QH1451 from Hanoi to Nha Trang, the flight time is 1 hour and 50 minutes, and the cumulative flight time is 5 hours and 55 minutes.