You don't have to wait. After flashing, reboot straight to bootloader and unlock the device.
My problem is, after I flashed the stock rom and re-lock the bootloader via fastboot, the OEM unlock function on my developer option was greyed out. So when I attempt to unlock again, the fastboot gives me an error message telling me to turn the OEM unlock function back on. I am not able to do so cause it is greyed out.