I threw the application at a large library on an external drive.
The process took more than 24 hours. After the 24-hour mark, the elapsed time rolled over to 00:00:00. Apparently, both the elapsed time and estimated time remaining VALUES include "days", but only the hours, minutes and seconds are displayed.
OS: Microsoft Windows [Version 10.0.19045.4894]
Video Thumbnails Maker version: 26.0.2