Pilgrimages could be short or long, with different sorts of people taking them up for different reason. Shorter pilgrimages to local shrines were more frequently undertaken by the lower classes and especially by women. They often sought simple cures and solutions to problems considered petty and small by their social superiors. Such journeys usually only took a few days at most, sometimes being short enough that the pilgrim could return home the same day.
Longer pilgrimages were dominated by the upper classes, especially men, though people from all levels of society undertook them. The three most famous destinations, Santiago de Compostela, Rome, and Jerusalem, brought the greatest benefits, but also required the greatest preparation and were the most dangerous journeys. What evidence we have of pilgrimage vows usually concerns promises to undertake this sort of journey and pilgrims arriving at such sites had often already received their miracle or had gone more out of religious devotion than a desire for something other than an experience in exchange.*
One result of the large numbers of people visiting these sites was the growth of an industry, or rather, several industries around pilgrimage. There was money to be made from pilgrims, in provisioning them, in caring for them on their journey, and in selling them souvenirs. The shrines they visited also acquired money through the donations of pious visitors, so having an effective, miracle-working relic, or even just one with a particularly holy reputation, could bring in significant income. Here lay the shadier industry: the acquisition and falsification of relics. This is not to say that pilgrimage shrines were rife with false relics or that the Church did not try to crack down thefts and false relics.** But pilgrimage was at times as much an industry as a religious experience.
*And then there were the Crusades, which were also considered pilgrimages. Urgh.
**Nor were all churchmen entirely scrupulous about such matters. My favorite story is the one about how in the 12th century, Bishop Hugh of Lincoln, after attempting to cut off part of the purported arm of St. Mary Magdalene, bit off a finger before anyone could stop him and took it back to Lincoln with him.
Webb, Diana. Medieval European Pilgrimage. New York: Palgrave, 2002.
Stalley, Roger. Early Medieval Architecture. Oxford: Oxford University Press, 1999.