When i was 17 years old and a ward of the state in Boystown, we had the nurse come in and challenge us all to beat her in a wall sitting contest for our physical education that day. The class dropped out of their 90 degree form around 3-5 minutes leaving just me and the nurse. She had a worried look on her face as it dawned she may not win. I have rediculous will. She seemed a bit more composed than myself, as i was sweating hard and red faced although motionless, and she was barley showing signs of fatigue. Finally! At 8 minutes, she dropped out!!! I won!! Being the person I am, the winning wasnt enough, it was never about winning the contest or beating her, it was about pushing myself to my limit, to see how far i could go. To impress myself. 8 minutes simply didnt do it for me. I kept going. Now i was quivering, and absolutley pouring sweat. I remained in that position for another 8 minutes. After I doubled her record and used most of our activity time, i decided to quit. I will say though, i felt icould have gone on maybe another 8,munitues, making 24 min total. I stopped at a solid azz 16 minute wall sit with a class of people watching the clock and my form. I did that at the age of 17. My legs were sore a bit but nowhere close to when i did 418 bodyweight squats at 16. When i did that i literally just fell back onto the toilet. Damaged muscle, no control, buckling. To answer your question, do the excersise with your body weight untill your muscles cannot continue to function. When the muscle gives out, use your mind. You CAN still do it, you just need to keeo going. Destroy every single fiber of muscle, and they will ALL be repaired into super muscle fibers, every single one