About 3 hours and 52 minutes.

According to Baidu Map, the distance from Jiayuguan to Dunhuang is about 374.8 kilometers, and the driving time takes about 3 hours and 52 minutes. It passes through: Lianhuo Expressway and Liuge Expressway. You can also take the D2737 train at Jiayuguan South Station at 18:44, and arrive at Dunhuang.

Get off at the station at 21:41 and the whole journey takes about 2 hours and 58 minutes.