It all depends on the amount of shrimp, water and if they are cool or half frozen. If you have 25/30 count shrimp completely unthawed and you were boiling 5 pounds. and you used 3 gallons of water it should cook them in about 2 minutes. Now if you are using an 8 quart pot you will cool the water down and then you should expect to double the cooking times suggested here. They have a rule you dump in the shrimp and when the water comes back to a boil they are done, but it all depends on the amount of water used.
Shrimp take a very short time to boil. Once you have removed the vein in the shrimp and cut it to your desired length, bring water to a boil. Drop in the shrimp and cook for about 4 minutes, until bright pink.
When cooking shrimp, it's important not to overcook this delicate crustaceans. Generally, small to medium sized shrimp should boil for 3-4 minutes, while large shrimp should boil for 5-8 minutes. Jumbo shrimp or prawns should be boiled for approximately 8 minutes. Shrimp will transform from translucent to opaque when cooked. Overcooked shrimp have a rubbery texture and lose their flavor.