The way you can tell when artichokes are done cooking and ready to eat or serve is when the leaf of the artichoke falls off easily.
Cooking or steaming artichokes take between 25 to 35 minutes to steam and be fully cooked.
After 35 minutes of steaming the artichokes you can check to see if the leaf falls off easily on the artichoke which it should and the artichoke should be cooked and ready to eat.
The way I like to make artichokes and team them is to squeeze lemon juice into some water and add 1 tablespoon salt; bring to a boil.
Then I put the artichokes in the steamer basket, stem-side up and then I cover the pot, and steam until the heart is tender when pierced with a tip of a paring knife, and inner leaves pull out easily.
For me it takes 35 minutes of steaming for the artichokes to be steamed and cooked fully.