Following graduation, about 40% of Biology majors will go off to medical school and about 30% will end up in biological graduate programs, though not all directly - many will take a few years to work in labs before starting graduate school. The remainder will do many things, including secondary school teaching, law school, business pursuits, volunteer work with the Peace Corps and the like. Duke Biology grads generally place well. For example, they have a rate of medical school acceptance that is twice the national average.
More information on possible career tracks for biology majors from Duke's Career Center.
Visit What Can I Do with this Major for more information on career options.