The scent of blood is one strong magnet for many animals, and one of these is the shark. Sharks have the ability to pick blood scent in very small quantities to the ratio of one part blood to 100 million parts water. This, mind you, is from a very long distances out. Sharks are built to pick up scent in two ways-diffusion and water currents
The first is a bit slow, involving molecules of blood dissolving in the water before spreading with more dilution taking place the further away from the starting point it gets. With the water current method, blood moves with water movement which makes it a faster way for scent to travel through water. The blood scent could be picked up by a shark in an interval of between one to a dozen or more minutes, depending on how fast the water is moving and how far away the shark is located.
A shark will be able to react instantaneously to the scent of blood if it happens to be near the vicinity of the blood spill. And you don’t need this to be from you. They can swim in relatively shallow waters to get to the prey, which explains the many cases of shark attacks near the beach front. This, however, will depend on the size of the shark.