get duration of audio file

I have made a voice recorder app, and I want to show the duration of the recordings in a listview. I save the recordings like this:

MediaRecorder recorder = new MediaRecorder();
recorder.setOutputFormat(MediaRecorder.OutputFormat.DEFAULT);
recorder.setAudioEncoder(MediaRecorder.AudioEncoder.DEFAULT);
folder = new File(Environment.getExternalStorageDirectory()
            + File.separator + "Audio recordings");
String[] files = folder.list();
    int number = files.length + 1;
    String filename = "AudioSample" + number + ".mp3";
    File output = new File(Environment.getExternalStorageDirectory()
            + File.separator + "Audio recordings" + File.separator
            + filename);
    FileOutputStream writer = new FileOutputStream(output);
    FileDescriptor fd = writer.getFD();
    recorder.setOutputFile(fd);
    try {
        recorder.prepare();
        recorder.start();
    } catch (IllegalStateException e) {
        e.printStackTrace();
    } catch (IOException e) {
        Log.e(LOG_TAG, "prepare() failed");
        e.printStackTrace();
    }

How can I get the duration in seconds of this file?

Thanks in advance

---EDIT I got it working, I called MediaPlayer.getduration() inside the MediaPlayer.setOnPreparedListener() method so it returned 0.