Originally Posted by
Maninthebar
So that implies that the question is not around how much firefighting there might be, but how long before landing MUST happen, which makes sense.
Correct. One of the first things ATC are considering is which is the nearest available airfield, and which is the nearest suitable airfield, not always the same thing and which airfields to potentially offer as divert options. Another factor is if the aircraft then has a radio failure, they have a rough idea of how long it can remain airborne, which airfields it may divert to.