Question: what is the chance of getting a legendary?
Answer: It's safe to say that for every 2 Super Magical Chests, or at most, 1000 chests, you will obtain 1 Legendary card. This is not a guarantee, however. The Free Chest is the most common chest of all. Since the chest currently has a 0.11% chance of dropping a Legendary card, you should get one for every 1000 Free Chests.