What makes a blood clot form in the body?

Variable. Usual clotting time in a test tube is 8-12 minutes. In vivo which is in a patient it depends on itissue='bruising or in a blood vessel where stasis or flow, trauma=injury, hypercoagulability affect the clot formation. Virchow's triad. It can be pretty quick in a sedentary person with risk factors such as cancer.
Several factors. Three things: increased coagulability (seen in cancer, pregnancy, hormone use, some inherited conditions, smoking); blood flow changes (such as stasis from lying around or sitting in the same spot for hours, turbulence from narrowing of blood vessels from atherosclerosis), and injury to the blood vessel wall (trauma, atherosclerosis, some vascular diseases).