As many as you wish. Students preparing exams study their lessons with phosphenes during long hours (sometimes up to 8 or 9 hours) and still feel the positive effects. For more than 40 years of diffusion of the method, no student has ever complained about any discomfort.
This is corroborated by the number of people satisfied by the results of their practice of Phosphenism. Thousands of people have practiced the method for the past 40 years. If it created any trouble, we certainly would have had to restrict spread to the Esoteric circle of people, whose thirst for knowledge takes them beyond such annoyances.
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I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.