How long will you bleed something bleed if you hit a vein?

Veins carry the deoxygenated blood to the heart do that it can go through the oxygenation process in the lungs. There is less pressure on the veins as they are further away (peripheral) from the heart and on the side of the heart coming from the body and into the heart. The arteries, on the other hand, carry oxygenated blood away from the heart to the body so that the cells in the body can use the oxygen and other nutrients to live. Now, regarding your question: It depends on several factors. First, whether or not you have normal or abnormal clotting factors in your body. These are generally manufactured and supplied by the liver and there are not just a few. Here is the whole clotting story, according to Wikipedia: http://en.wikipedia.org/wiki/Coagulation

Secondly, whether or not you have an adequate supply of platelets in your blood. These are microscopic fibers that are attracted to areas of bleeding. They form a "clot" on the bleed and cause the bleeding to stop.

If both of these coagulation systems are normal and, depending on how large the "vein" is that is bleeding, you should stop bleeding fairly quickly (inside of ten to twenty minutes) if you do nothing but let the wound bleed. If you are not suicidal though and have any common sense at all, you would simply apply pressure to the bleeding area and the bleeding would stop even quicker as long as you hold pressure there.

If there is an artery involved, the bleeding could last much longer. The reason is that there is significantly more pressure on an arterial bleed than on a veinous bleed. This keeps the platelets from doing their job. It would be like trying to stop a garden hose with a fireman's nozzle on it. The pressure would not allow you to stop it quickly so you'd have to stop the flow at the source. Again, you'd want to hold significant pressure and get to the nearest ER so they could stop the bleeding for you.

Luckily, the veins are closest to the surface so you're most likely to hit a vein rather than an artery but arteries are knicked fairly often.