Considering that the original timeline was for Fir to ship 6 months after Evergreen, I’m predicting that it will be 12 months.
If it were just an issue of producing a new PCB for the i.MX 8M Plus, then 6 months is probably doable. The big question for me is whether Fir requires any special mainline Linux kernel work. The i.MX 8M Plus has added a bunch of new components (digital signal processor, neural processor & hardware video encoder) and changed the secondary core from Cortex-M4F to Cortex-M7, so I bet that it will require some kernel work, and NXP has been lackluster in getting its i.MX 8 chips supported in mainline Linux.