Apparently eighteen months, according to the new book Forbidden Fruit: Sex and Religion in the Lives of American Teenagers, written by Mark Regnerus, professor of sociology at the University of Texas in Austin and research associate for the National Study of Youth and Religion. says that Regnerus’ conclusion is fairly simple:

What really matters is not which religion teenagers identify with but how
strongly they identify. After controlling for all factors (family satisfaction,
popularity, income), religion matters much less than religiosity. Among the mass
of typically promiscuous teenagers in the book, one group stands out: the 16
percent of American teens who describe religion as “extremely important” in
their lives. When these guys pledge, they mean it.

World Magazine’s blog also comments on the article. The Weekly Standard also comments on the book.