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
When did Mumtaz Mahal become empress?
Mumtaz Mahal became empress in 1628

Question: How long did she serve as empress?
Self-contained rewrite: How long did Mumtaz Mahal serve as empress?