Lobster tails should boil for no more than 10 - 12 minutes, depending on their size. The meat should be a bright white, and have no hints of gray in it when it is cooked properly. The shell should be a vibrant reddish orange color and the juice from the lobster tail should drip from the lobster tail clear, with no cloudy nodes to it.
When boiling lobster tails, here's what you do: you drop the tails in boiling water, bring back to a boil, lower heat and cook for 3-5 minutes for 1-3 ounces, 5-7 minutes for 4-6 ounces, 10-12 minutes for 10-12 ounces, and 15-20 minutes for more. Do not over boil as these will become very soggy.