Duration problem happen for all video or just one ?

Check the templates_c/debug.txt file and see what duration it says. You may delete the file before uploading a video, so the file will only contain details of the file you just upload or it will include all video upload and other load details and it will be difficult to find the information.

In cpanel, there is a File Manager. After login, check the buttons/link, you will be able to find it. Its better to use FTP software like "FileZilla" for managing files. You should set your FTP mode to binary before uploading any files or it will not work.