A blood clot can travel from the leg to the lungs very rapidly once it breaks loose. The exact speed varies widely depending on factors such as the size of the clot, the individual's health, and circumstances of clot formation. In some cases, a clot can reach the lungs within minutes or hours after detaching from the leg vein, while in others it may take days or weeks

. The risk of a clot breaking off and traveling is highest within the first four weeks after the clot initially forms, as it is most fragile during this period

. Once a clot travels to the lungs, it can cause a pulmonary embolism (PE), which is a blockage in the lung arteries and can be life-threatening if not treated promptly

. In summary, although there is no precise timeframe for how fast a clot travels from the leg to the lungs, it can happen very quickly—potentially within minutes to hours—and requires immediate medical attention if symptoms of PE appear