You can use Duet Web Control or PanelDue to simulate printing a file, which will give you an accurate print time. In Duet Web Control, Go to the GCode Files page (or GCode Jobs if you are using DWC 2.0beta), right click on the file you want, and click Simulate. When the simulation completes it will tell you how long it will take to print. It also writes the simulated time to the end of the file so that it can be retrieved later.
PanelDue and DWC 2.0beta display the simulated print time as part of the file properties. DWC2.0 beta also uses the simulated time to give you an estimated print completion time when you come to print the file.