Conversation so far:
What is Mughal empress?
Shah Jahan designated Mumtaz as his chief empress with the title of 'Malika-i-Jahan' ("Queen of the World".
Where was she the empress?
CANNOTANSWER.
When did she become empress?
1628.

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