To add some historical context this is how long it took GrapheneOS to get on the past two main phone series (Pixel 7 and 8). Note, I'm not including A series, the Fold, or the Tablet, since they are based on the main phone series, which generally means it was even quicker to port.
Pixel 7 - Released October 13, 2022 - First GrapheneOS release on October 16, 2022. Just three days.
Pixel 8 - Released on October 12, 2023 - First GrapheneOS release on October 23. Just eleven days.
Of course this is no guarantee for how long the 9 series would take to port. But based on history, it seems likely the GrapheneOS developers will do it very quickly.