<p>What is the simplest way to determine the length (in seconds) of a given mp3 file, <strong>without using outside libraries</strong>? (python source highly appreciated)</p>

You can use
pymad
.  It's an external library, but don't fall for the Not Invented Here trap.  Any particular reason you don't want any external libraries?
```
```
import mad

mf = mad.MadFile("foo.mp3")
track_length_in_milliseconds = mf.total_time()
```
```
Spotted
here
.
--
If you really don't want to use an external library, have a look
here
and check out how he's done it.  Warning: it's complicated.