According to Rashi, the word, “kazos” (“like the time”) is in the future tense. Accordingly, Mordechai is reminding Esther that there is no surety in her remaining queen in the future. For all she knows, Achashverosh will get rid of her in the same way he got rid of the last queen. If that would be the case, her effectiveness in defending the Jews has a potential expiration date.
According to Ibn Ezra, Mordechai is emphasizing to Esther that this very occasion is the reason why she is in the royal position in which she finds herself.
The Ohel Moshe writes that we are all here for one moment, to act in a way that will glorify H-Shem. We are all Divinely placed in the positions in which we find ourselves for a reason – whether we understand that reason, or not.
R’ Dovid Feinstein writes that Jews in prominent positions should realize that they are only there to make a Kiddush H-Shem.
Along the same lines, Vilna Gaon writes that Mordechai is telling Esther that her refusal will not just forfeit a reward, but will also be punished for being responsible for the deaths of the Jews.
The Malbim writes that H-Shem always has a set time to rescue the Jews, and Mordechai is telling Esther that this is the time to join in the rescue.
According to the Me’am Loez, Mordechai is pointing out that the Jews living in Persia might despair over time, and lose faith in their redemption.
Another opinion he brings is that this matter is time-sensitive, as Achashverosh may not have time to send messengers to recall the decree over his giant kingdom. G-d Willing, we will see in the final chapters of Megillas Esther that this concern was legitimate.