The following is from a medical text book called: "The first principles of Gastroentiology" by E A Shaffer.
1.3 Natural History of Gallstone Disease page 443
Gallstones grow at about 1-2 mm per year over a 5- to 20-year period before symptoms develop. They frequently are clinically "silent," being incidentally detected on routine ultrasound performed for another purpose. Most patients (80%) with Gallstones never develop symptoms. Problems, if they do occur, usually arise in the form of biliary pain during the first 5 to 10 years.