For how many days did Xerxes display his riches before his seven-day feast in Susa?

Answer: 180 days

For a full 180 days Xerxes displayed the riches of his kingdom, then held a seven-day feast for everyone in Susa.

“And for a full 180 days he displayed the glorious riches of his kingdom and the magnificent splendor of his greatness. At the end of this time, in the garden court of the royal palace, the king held a seven-day feast for all the people in the citadel of Susa, from the least to the greatest.”— Esther 1:4-5 (BSB)