900 to 1200 years ago, in Tibet and India, was the heyday of Mantrayana. Note that "retreat" is rarely mentioned in the biographies of the 84 Mahasiddhas (see Masters of Mahamudra). Now I'm not knocking retreat but here are some interesting facts. (1) Three-year retreat is a relatively recent invention, from the 19th century. It was meant to preserve traditions, by providing intensive training in the formal aspects of practice. In other words, a one-size-fits-all approach. But Buddhism, and Mantrayana, was never a one-size-fits-all religion. More like a "grow your own garden" of methods religion. (2) Retreat is little mentioned in the life-stories of the 84 Mahasiddhas. Many of them lived ordinary lives, with little personal means. And yet these Mahasiddhas, among others, are largely the source of Tibetan Buddhist lineages. (3) Three-year retreats in the west mostly train people how to go through the motions of practice. They do not teach how to integrate practice with ordinary life. (4) Short expensive retreats in America or Europe mostly cater to beginners. No doubt they are helpful but they do not necessarily substitute for studying a long time with a teacher or teachers. Moreover, once one has a teacher, devotion and guru yoga are the fastest approach to liberation. For devotion's sake, it is not necessary to be near a teacher or be in a retreat. It might be better, for devotion's sake, to spend periods far away from one's teachers and facing difficult life-situations.
With all due respect to practitioners and retreatants who have spent many years practicing hard, I think the Mahasiddha models are better suited to our times. Like most of the old Indian adepts (and bearing in mind, 84 is just the number of recorded stories, not the actual number of accomplished ones), we have limited means and limited time for study. Therefore the approach of the old Siddhas might teach us something practical. The lessons I've drawn from those stories is that it is more important to have intense faith and devotion for a teacher, than to spend a lot of time or money in retreat, or to study formally with one teacher. Also, to have frequent contact with a teacher -- the more humble, less famous and less busy, the better -- is enormously beneficial. But above all, to have renunciation and bodhichitta, along with a genuine sense of empowerment from a lineage full of blessings, and a strong guru yoga practice, is all but guaranteed of success.
Nowhere in any scripture that I am aware of does it say, "you must do retreat". Make your life a constant retreat -- this was the advice of Kunu Lama. He was a very humble practitioner who never drew attention to himself, had no titles and no heroic qualifications of having spent many years in retreat or gathering academic titles and distinctions, had no monastery, and no retinue. Yet he became, towards the end of his life, a very highly regarded Dzogchen meditator and scholar of Buddhist teachings. HHDL and some of the Drikung Kagyu lamas of our acquaintance were among his students.
I admit it is easy to say this now. When I was younger and had more time and money, I traveled for retreats and met lots of different teachers. Looking back, at best, those experiences are mostly ornamental. The essentials -- which anyone can practice anytime, anywhere -- are most important, and sufficient.