The 20 min limit only applies to cached recording where you decide the last 20 min of play is worth keeping and activate the replay mode after playing that segment, which saves it to a written file. This is why it's called "Shadow" play, the 20 min segment sits in cache like a shadow copy ready to be written to file.
If you want to record more than 20 min you have to put it in manual record mode and have it recording in the background as you're playing. Unfortunately this way you often end up with capture files taking up space that you have to delete if you decide the gameplay wasn't to your liking and want to play through it again.
Raptr's Game DVR works similarly, and is compatible with certain models of AMD or Nvidia GPUs, but...