By His Holiness Shrila Pramod Puri Goswami Maharaj in the book Essence of
Devotion, first chapter:

"Thereafter he constructed a tank by the side of this Govardhana Hill which
he named as Radha-kunda. It was at this very place after building his place
of devotions in the form of a small hut, by the side of this Radha-Kunda
that he began to exhibit intense renunciation. He then demonstrated the
method of intense devotions by taking earnestedly three lakhs of Holy Names
daily and completed the vow of chanting 1000 million Names (1 billion)
altogether at that place. E

"Even after the successful completion of that vow, Sri Srila Prabhupada
maintained always the practice of chanting the Holy Names seriously intact.
He especially gave stress to the process of chanting the Holy Names as top
priority in devotional practice, being himself attached to this method.
Wherever I would raise the proposition to Sri Srila Prabhupada whether I
could reduce my chanting somewhat in respect of my schedule, he would reply
insist instead that everyone should attempt to complete one lakh of Holy
Names as a daily function.

"He used to remark gravely that Sriman Mahaprabhu would not even accept
water from one who did not take to chanting one lakh of Holy Names daily.
Thus his counsel stood as all would have to find time irregardless, to
complete this chanting one lakh of Holy Names without fail as a daily
practice.

"He added however that to simply complete that amount was not the main
factor, but one had to strive to remain attentive throughout such chanting
and try to keep clear from the offences to the Holy Names.

"Those who had really the slightest regard for Sriman Mahaprabhu would never
in any way neglect His words."