When decoding, for instance, a variable-bitrate MP3, gstreamer reported durations are, to say the least, estimates. I’ve tried to get a better result in a few ways. First off, some files yield a duration tag, but even if you’re lucky and it is there, there are no guaranties about precision. After that I tried seeking to the end (GST_SEEK_END) of the stream and querying the position, which gstreamer didn’t like. Finally, routing the audio into a fakesink, waiting for the end of stream and then querying for the position gives the right result. It’s not the prettiest method, but it works.
This is a Python script that prints the duration of a media to stdout.