Shrimp is extremely easy to overlook, and that is what makes it rubbery. The second it turns pink, take it off the heat. Dont worry about undercooking, just overcooking. Depending on size, it could be as little as less than a minute. The biggest shrimp, the huge ones that look like small lobster tails (and are very hard to find and expensive, so you won't cook with them very often, if ever) still only take a few minutes.