How many years had the prophetess Anna been married before she was widowed?

Answer: Seven

Anna had been married for seven years and then lived as a widow, worshiping night and day with fasting and prayer.

“There was also a prophetess named Anna, the daughter of Phanuel, of the tribe of Asher, who was well along in years. She had been married for seven years, and then was a widow to the age of eighty-four. She never left the temple, but worshiped night and day, fasting and praying.”— Luke 2:36-37 (BSB)