It’s a common problem that most players can’t determine the total time of an MP3 file when it’s opened. 
Generally, the players read the bit rate of the first frame and estimate the total time from that bit rate and the file length. Since many audio files begin with silence, the first frames are often encoded with a very low bit rate, which leads to a much longer estimated total time. This time is often corrected while the file plays back, once a better estimation becomes possible. Some players will store the corrected length, so the time is displayed correctly upon opening the file again.

This problem does not occur when creating MP3 files with a constant bit rate (CBR). You can select this option in the “Quality” field in the export settings.