A stretch should be held in a relaxed position where you feel a slight pull in the muscle while stretching and the pull should be gone after 30 seconds.
After the first 30 seconds you can try and lengthen the muscle again and hold for another 30 seconds, only if it's not painful.
You should not strain by holding your breath, forcing the position or feeling pain.
Always stretch both sides of the body.
QUAD STRETCH (QUARDICEPS)
Sit on the floor (or on a bed if you cannot get on or off the floor) and lean back. The further you lean back and bring your foot back the more of a stretch you will feel.
The idea of this stretch is to lengthen the left legs thigh in this picture.