It can take a long time. Research shows that the average couple (under 35) has only a 20 to 30 percent chance of getting pregnant in any given menstrual cycle, as long as they have regular sex1.
Studies show that in general, about 84% of couples will conceive within 1 year if they do not use contraception and have regular intercourse. Of those who do not conceive the first year, about half will do so in the second year (ie., 92% will get pregnant in 2 years)2.
All of these numbers assume that the couple must have regular sex.
1. Dunson, David B., Baird, Donna D., Colombo, Bernardo "Increased Infertility With Age in Men and Women", Obstet Gynecol 2004 103: 51-56
2. Guidelines of the Royal College of Obstetricians and Gynaecologists via US National Guideline Clearinghouse