How long did Esther ask the Jews of Susa to fast for her?

Answer: Three days

Esther asked them not to eat or drink for three days, night or day, and she and her maidens fasted too.

““Go and assemble all the Jews who can be found in Susa, and fast for me. Do not eat or drink for three days, night or day, and I and my maidens will fast as you do. After that, I will go to the king, even though it is against the law. And if I perish, I perish!””— Esther 4:16 (BSB)