Part of Portal 2 · Portal 2: In Motion
Portal 2: Peer Review includes an additional course for Portal 2 co-op campaign, dubbed "Art Therapy"; as well as Challenge Mode for both single-player or multiplayer campaign, where players can play specific levels and compete in friends/worldwide leader-board.
Verdict
<1week
You'll breeze through this one.
Most players who finish the story can polish this off.