You will find mentioned in many Vedic scriptures that maha-mantra is especially recommended for understanding God in this Age of Kali Yuga, such as Agni Purana, Kali-Santaram Upanisad, Brhan Naradiya Puranam, and many others. Haridasa Thakura chanted 3 lakhs of names each day, that means 16 names in the mantra, 108×16 or 1,728 names in each round of mala, dividing 300,000 by 1,728 comes to roundabout 175 rounds on beads daily. And for us it is even difficult to chant 16 rounds! That is why Haridasa Thakura is “namacarya.” We find in Caitanya Caritamrta information that both Lord Caitanya and Haridasa Thakura were constantly absorbed in chanting the Hare Krishna mantra: Hare Krishna, Hare Krishna, Krishna Krishna Hare Hare / Hare Rama Hare Rama Rama Rama Hare Hare.
From Srila Prabhupada’s letter to: Yajnesvara — Bombay 2 January, 1972