Creating a USB install disk for macOS (OS X) can take 5 to 40 minutes and the createinstallmedia utility doesn’t provide any indication of the progress. However, the Disk tab in the Activity Monitor application (in Applications → Tools→ Activity Monitor) does provide a real-time view of how much data the tool has written to the USB disk which is a good estimate of the overall progress:
Open the “Get Info” for the USB drive being written. It will show how much of the drive is used, which is a good measure of progress.