Just how fast can a pus filled fistula tract completely destroy a lot of bone around a tooth?

Depends: A fistula that is draining pus is usually due to infection from the tooth or gum disease. Usually it is chronic & your immune system will try to contain the infection. Then the bone loss is gradual. When your immune system is weakened from a cold, flu, or other causes, the bacteria inside the site will proliferate and the bone loss is accelerated. Best to have the problem fixed by your dentist.

Quickly: Depends on whether it is a chronic or an acute infection. No infection should go untreated. Please see your Dentist ASAP.