A whole head of cauliflower needs about 25 minutes of cooking time when set in1 inch of boiling salted water.  Usually the first 5 minutes is cooked without a coves and then the cauliflower is finished with the lid on.  Alkaline or hard water can turn the cauliflower an unappealing yellowish color so some chefs prefer to add a little acid (lemon juice) to keep the brilliant white color.  Care should be exercised in cooking as overcooked cauliflower becomes very mushy and unappetizing.