It is a common problem with MP3 files that are stored with a variable bit rate (VBR), that most players can not determine the total time of the file when it is opened. Normally they just read the bit rate of the first frame and estimate the total time from that bit rate and the file length. As many songs begin with silence, the first frames are often encoded with a very small bit rate, which leads to a much longer estimated total time. Often the time is corrected while the file is played back as a better estimation becomes possible. Some players will store the corrected length of files that they have already played back, so the time is displayed correctly when you open the file again.
This problem does not occur when creating the MP3 file using a constant bit rate (CBR), which you can choose in the Quality selector in the Track-Export-Settings.