Well, here's the physiologist answer to start: it depends. There's several dozen factors involved in blood clotting, and all of these are expressed in very slightly different levels in different individuals. Because of the number of factors, there's a wide variability in clotting speed. From a clinical standpoint, there's several tests used to assess clotting time, and each of these tests reflects on some of the various clotting factors I alluded to above. The bleeding time is a pretty straight forward test - the patient's skin is cut (taking care to avoid all large arteries, veins, nerves, etc). The time for the bleeding to stop is measured. Depending on the dimensions of the cut, the bleeding should stop in around 2 minutes but sometimes as long as 9 or 10 minutes. This test measures Platelet based clotting mechanisms. There's another common method used - called the PTT test. PTT stands for partial thromboplastin time. This measures some of the other factors that exist in the blood. In this test, blood is drawn with a needle and taken to a lab. While they're carrying it to the lab, the blood is mixed with clot - stopping compounds. When the blood gets to the lab, the anti-clotting drugs are neutralized, and the blood is timed until it clots. This takes between 25 and 39 seconds (these values may vary slightly depending on what textbook you read). So depending on how you look at clotting, it can take a few seconds or a few minutes. But as they say in field medicine - "all bleeding stops eventually!" --SP