Kneeling on rice for an extended period is very painful. I would have stopped it a long time before I actually did had there not been a Mistress on a webcam telling me to keep kneeling and adding an extra minute every time I moved.

Toothpaste is a less painful but still as irritating equivilent to deep heat/ben gay. It hurts as it dries and then hurts again as you wash it off. Works on nipples and genitals.

However, I agree with the others - to find your limit you will need someone else to monitor the process, telling you to stay in position and not stop. Maybe someone would be willing to webcam session with you?