Esther in Fausset's Bible Dictionary

The Book of Esther describes in the same year, the 3rd, the lavish feasting during which Vashti was deposed, 488 B.C. In his 7th year the battles of Plataea and Mycale, according to secular history, drove Xerxes in fright from Sardis to Susa. So, in Scripture, it was not until the tenth month of this 7th year that Esther was made queen. The long delay between Vashti's deposal and Esther's accession is satisfactorily accounted for by the Greek expedition which intervened. On returning from it Xerxes tried to bury his disgrace in the pleasures of the seraglio (Herodotus vii. 35,114); as indeed he had begun it and, according to Herodotus, at intervals continued it with feastings. Possibly Vashti answers to the Amestris of secular history, who was queen consort from the beginning to the end of his reign, and was queen mother under his son and successor Artaxerxes. Esther cannot be Amestris, since the latter was daughter of a Persian noble, Otanes; if Vashti be Amestris, then her disgrace was only temporary. Or else Vashti and Esther were both only "secondary wives" with the title "queen." A young "secondary wife" might for a time eclipse the queen consort in the favor of the king; but the latter would ultimately maintain her due position. Esther's influence lasted at least from Ahasuerus: 7th to the 12th year and beyond, but how far beyond we know not (Esther 3:7; Esther 3:10). His marriage to a Jewess was in contravention of the law that he must marry a wife belonging to one of the seven great Persian families. But Xerxes herein, as previously in requiring the Queen Vashti to appear unveiled before revelers (such an outrage on oriental decorum that she refused to come), set at nought Persian law and prejudice...

Link: https://bible-history.com/faussets/E/Esther/