Megillas Sesarim says Esther was arguing here that her not being summoned in the last thirty days was indicative of the fact that she soon would have to be requested.
R’ Yaakov of Dubno gives the opposite answer – since Achashverosh had not called her in thirty days, Esther feared that he had lost interest in her.
Kisei Shlomo writes that Esther realized that the Jews’ salvation would really not come from her, but through their own teshuvah. She therefore picked the large number of days to buy time for the Jews to repent on their own.
The Maharal quotes from the Talmud (Brachos 58b) that a person who is reunited with a friend after thirty days says the blessing of Shechechiyanu. The Maharal explains that the reason for this is the intense joy brought about by the cessation of the absence. In other words, Esther emphasized that she had not been summoned for thirty days to clarify that when Achashverosh will actually summon her after so long a separation, his emotional state will be far more amenable to her suggestions.
According to Rav Yitzchak Hutner, this entire conversation justifies the custom of drinking on Purim ad d’lo yada, up to the point that one does not know the distinction between “blessed is Mordechai” and “cursed in Haman” (Talmud, Megillah 7b). He explains that, during the rest of the year, we are warned against digging deep into the secrets of the world, like the material from which the Earth was made, the symbolic meaning of Yechezkiel’s vision of H-Shem’s chariot and retinue, etc. On Purim, however, the point of drinking is to break down our inhibitions, open our minds, and reach levels of intellectual understanding to which we are not usually privy. On Purim, too, we are able to enter the courtyard of the King, and He will allow us to comprehend that which we were otherwise not invited to understand.