It depends, there are many different causes. The redness you see is an inflammatory response. The blood vessels in the conjunctiva dilate so that more blood can get to the eye and deliver leukocytes or immune cells. There can be many causes, the picture you posted looks like iritis, an inflammation of the iris and surrounding structures. But corneal abrasion, corneal ulcers, conjunctivitis, viral or bacterial, allergies, endophthalmitis, embedded foreign bodies, dry eyes, and uveitis can all be causes of a blood shot eye. The most common cause is what's known as a subconjunctival hemmorhage. This is when a blood vessel in the conjunctiva pops leaking blood into the conjunctival tissue. The blood moves around dependent on the person's position and the gravitational effect on it, and usually heals in about 2-4 weeks depending on how long it takes the body to reabsorb the blood.