that depends on the quality of the mp3s, and the official specs usually go with a lower bitrate to boost their specs.
also, the sizes are shown in base 10 while the files are in base 2, which means a bit of space is "lost," again due to advertising
assuming you have only 320kbps CBR MP3s encoded in LAME (highest quality MP3 you can get)
a 5 minute song will be around 11mb
a 7 minute song will be around 15-16mb
there is also the firmware size to worry about
for 5 minute songs
8gb will hold around 710
16gb will hold around 1440
32gb will hold around 2870
for 7 minute songs
8gb will hold around 530
16gb will hold around 1045
32gb will hold around 2100
this leaves quite a bit of leeway, as I calculated each to leave around 400mb for the firmware and apps, since I'm assuming this is mainly for music, partly for apps.