Hare Krishna! For me, it was very quick. I was somewhat familiar with and interested in Krishna consciousness for a few years, having read Bhagavad Gita and other texts, but I was preoccupied with other things, and mostly atheistic. But when I visited a temple for the first time during a lull in my life, I wanted initiation as soon as possible. I began chanting sixteen rounds, and visited another temple where I met my spiritual master. He said if I wished to seek initiation, I should continue chanting sixteen rounds a day, and in a year’s time I could take the next step if I was ready. So, I kept chanting and growing in devotional activites over the year, and grew ever more eager and certain. It was an instinctive desire, but also one built on knowledge and practice, and that year of development was invaluable.
As for caffeine, onions and garlic, etc, these are not mentioned in the vows or four regulative principles, but many find it helpful to avoid consuming them. In my opinion, dietary concerns are often given more attention than is needed, while there is much else which ought to be focused on when developing individual spiritual progress and preaching to others. If it is useful to a devotee to strictly follow a sattvic diet, that is wonderful. But if a devotee drinks a cup of coffee, what use is there in gossip and browbeating? I think it is far more important to focus on positive service, in yourself and others, and what we can proactively do in the movement, rather than to obsess over who is eating what. Adhering to standards of dietary purity, cleanliness, fasting, etc is meant to be an aid to spiritual progress, not a practice unto themselves. They are secondary concerns. The number one concern is Krishna. Always, Krishna. If a person would rather go to Starbucks than chant their rounds, that is where the problem lies. But if a person would rather nitpick over the details of their diet, all while forgetting the whole reason for it (Krishna! Krishna!), and find it difficult to concentrate on their chanting, I would say that person, too, should focus less on the gross physical body. Krishna is more interested in your sincere love and devotion than a standard which shall always come up short in the material world in Kali Yuga. Love Krishna first, and you will be purified. Haribol!