Blizzard
No specific details on timing at the moment, but we are planning to allow the challenge to be attempted at any time, regardless of the status of the Mage Tower, as we approach the end of Legion.
As said, nothing on timing to share at the moment, but we will share more details when we have them. For now, you can continue your attempts, get those strategies down, and work towards better gear to help you complete the Mage Tower!