You should be using an AI from the first day of the cycle. It is much easier and healthier to prevent side effects, rather than deal with them once they take place. Therefore, use the AI from the first day of the cycle. And by the way - gyno is not the only side effect of high estrogen, so you really should not regard it as a reliable indicator of high estrogen.
You take them BEFORE gyno symptoms start. That is the whole point of an AI. To prevent side effects from occurring. If you wait until the side effects occur, it is already too late.
Start your AI on day 1 of your cycle and run it throughout your entire cycle and PCT.