Conversation so far:
What is Mughal empress?
Upon his accession to the throne, Shah Jahan designated Mumtaz as his chief empress with the Mughal empress title of Queen of the World and Queen of the Age.
Where was Mumtaz Mahal the empress?
Mumtaz Mahal was the empress of the Mughal Empire

Question: When did she become empress?
Self-contained rewrite: When did Mumtaz Mahal become empress?