Nowadays we frequently have the impression that 16 rounds are some kind of eternal standard and it was always this way, but actually Vaishnavas in previous centuries had different standards for Japa.

In the times of Sri Caitanya Mahaprabhu, the minimum standard for Vaishnavas (followed even by householders) was to chant one lakh or 100,000 names. This was taught by Mahaprabhu himself, as narrated in the Sri Caitanya Bhagavata:

“As the devotees invited the Lord (Sri Caitanya Mahaprabhu) to take His meal in their houses, the Lord took the opportunity to teach them all this topic. When invited to take a meal, the Lord smiled and said, “First you should become a laksesvara”.
“I take meals only in the house of a laksesvara.” Hearing this, the brahmanas became worried.
The brahmanas offered prayers to the Lord, “O Gosaani, what to speak of a hundred thousand, none of us possess even a thousand. If You don’t accept meals from us, then let our entire household be burnt to ashes.”
The Lord replied, “Do you know who is a laksesvara? He is someone who daily chants a hundred thousand holy names. I call that person a laksesvara. I take meals in only such a person’s house, not in the houses of others.”
Hearing this merciful statement of the Lord, the brahmanas gave up their anxiety and became joyful. “O Lord, we will chant a hundred thousand names. Please therefore take Your meals in our houses. It is our good fortune that You are teaching us in this way.” (Caitanya Bhagavata, Antya, 9.116-125)

One lakh of names equals 58 rounds (16 names per mantra, 108 mantras per round) but generally 64 rounds are taken as one lakh. Many devotees at those times would make vows to not accept prasadam before completing their chanting. Even householders would follow this standard.

Renunciants would frequently chant much more. We all hear the example of Srila Haridasa Thakura chanting 3 lakhs, but some accounts describe that Sri Caitanya Mahaprabhu himself, as well as many of his associates, would also chant 3 lakhs. In the Jaiva Dharma is described that many of the renunciants that are mentioned in the book were chanting two lakhs. It’s described for exemple that Lahiri Mahasaya started chanting two lakhs imediatelly after being initiated, following the exemple of the Vaishnava Babajis he was living with.

By the time of the Gaudiya Math, Brahmacaris, and Sannyasis were expected to chant 64 rounds, while householders would have 16 rounds as an absolute minimum. Srila Prabhupada himself was chanting 64 rounds during his renounced years in Vrindavana, before coming to the west (despite all his literary work).

Srila Bhaktisiddhanta Sarasvati Thakura was chanting 3 lakhs as a minimum for 9 years. He made a vow of chanting 1 billion names before starting his preaching and took 9 years to complete it. During his time he was doing great austerities, like eating just rice once a day and he was so absorbed in chanting that he would not even take the time to fix the holes on the root of the hut he was living in. After concluding this vow, he continued chanting one lakh as a minimum until his final days, just as he was instructing his disciples to do.

When Srila Prabhupada came to the West, he intended to establish 64 rounds as the standard for Brahmacaris and renounced devotees, following the example of the Gaudiya Math, but due to veement protests from the devotees he eventually agreed to lower the standard to 16 rounds, but “no less”. This is narrated in The Hare Krishna Explosion:

“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.” (p. 63)

He comments on that on the Nectar of Instruction and a few other references: “The Krsna consciousness movement prescribes sixteen rounds daily because people in the Western countries cannot concentrate for long periods while chanting on beads. Therefore the minimum number of rounds is prescribed. However, Srila Bhaktisiddhanta Sarasvati Thakura used to say that unless one chants at least sixty-four rounds of japa (one hundred thousand names), he is considered fallen (patita). According to his calculation, practically every one of us is fallen, but because we are trying to serve the Supreme Lord with all seriousness and without duplicity, we can expect the mercy of Lord Sri Caitanya Mahaprabhu, who is famous as patita-pavana, the deliverer of the fallen.” (Nectar of Instruction, text 5)

We have the impression that all devotees chant 16 rounds, but this is actually not true. I personally know many devotees who chant more. Some chant 20 rounds, or 32, a few chant 64 rounds. However, usually devotees who chant more are very secretive about that, because most devotees simply can’t understand how someone can chant 64 rounds and just become disturbed in hearing it. I also know cases of devotees who chat more on Ekadasis and especial days, up to 192 rounds. All of this happens around us, although we are often not aware of it.

Chanting 64 rounds may seem something from another planet, but if one concentrates it’s possible to chant in around 5 hours, and these can be broken into segments. In the beginning of our movement, devotees were trained to always carry their beads whenever they would go, so they would always remember to chant whenever there was free time. If we get the habit of always chanting when there is opportunity we can chant a lot of extra rounds by the end of the day. Of course, it’s not always possible to chant on beads because of different circumstances, but a counter can help on that. Chanting more is just like fasting on Ekadasis and other practices, it may sound scary at principle, but when we finally start doing it we see that it’s not so hard.

The whole idea is that we should start with a certain amount of rounds (even if just one round), and gradually increase during our lives, both in quality and quantity. Quantity also helps, because the purifying effect of the holy names is cumulative and we tend to start chanting better as we chant more. 16 rounds are just the bare minimum, not a state of perfection.

We often think that as long we are doing different services we don’t really need to chant, but Prabhupada mentioned that 90% of our advancement is due to chanting, and nothing else. Different services, as well as the study of the scriptures and other limbs of our devotional practice augment our chanting, but don’t replace it. The idea is that by doing services and studying we can continue to remember Krsna even when we are not chanting, and in this way chant better. To just do services and not chant is like practicing a skill but never really using it.