you have filesize and bitrate

The file we'll use for this is 8,192,000 bytes

Take the filesize and divide by 1024. You now have KB
say its a 8000KB file @ 192kbps
Make 192 kb into KB [divide by 8]. This is 24
Take a calculator. Divide 8000 by 24. Approx 333
This song is 333seconds long, or 5:33

Formula: s = (f/1024)/(b/8)
///[({ s = seconds, f = filesize[bytes], b = bitrate[kbps] })]\\\

also, the one with the highest score is generally your best bet.