It cannot be a rocket science, though
equal or longer than 5000 millis for ANY crystal value
the delay() and millis() aligned in a way it will not be dependant on the crystal frequency
Frankly, I do not understand why the delay() and millis() shall be dependant on a crystal frequency entered in boards.txt.
My understanding is for the clock frequencies from 32KhZ to 20MHz one shall be able to compile a delay() and millis() where one millis() unit is very close to 1ms