How Srila Prabhupada fixed Chanting rounds as sixteen?

The Hare Krishna Explosion, by Hayagriva dasa says:

When Srila Prabhupada was instructing the number of rounds of chanting, this discussion happened between him and disciples:

"Now that you have beads," Swamiji says the next morning, "you should chant sixty-four rounds every day."

"Sixty-four rounds? To pronounce every word of the mantra distinctly, we require five minutes to chant a round. Sixty-four rounds would take over five hours. 

"Impossible!" we say. "We'll never have the time."

"All right," Swamiji says. "Thirty-two rounds."

"Impossible," we say. 

"We'll never be able to do it. It's way too much."

 "All right," Swamiji says. "Sixteen rounds. No less."

 Thus, it was fixed as 16 rounds.