title: ExoPlayer getDuration
body: <p>I'm trying to play mp3 file from url with <code>ExoPlayer</code>. </p>
  
  <p>Everything is fine but now I wonder when is the time it's safe to <code>getDuration()</code> of the audio track. I just need it once track is loaded. Didn't find it in google's example project.</p>
  
  <p>When I try to get it immediately after <code>exoPlayer.prepare()</code> then I get <code>UNKNOWN_TIME</code>.</p>