Lower yourself over the top and start feeling the object going in deeper, making you wider till it got stuck.
In these 20 minutes, you may please and edge yourself but you may not come! If you can't hold the pose you may sit on your knees, but the object will maintain in that hole at all times. Make sure that every 4 minutes you try to push yourself even further over this huge object that is stretching every part of your butthole.
At the end of he 20 minutes, mark the object somehow so you now how far it was inside you.