(Strictly speaking, a comet can have both a type I tail - ion tail, and a type II tail - a dust tail. This question is about the type II tail - the dust tail).
Now I understand that some of the source of the dust is volatile materials in the comet oxidising due to the comet's proximity to the sun. But surely some of these comets have been orbiting for hundreds of thousands of years (if not millions). Why do they still have unspent fuel? Why are they still dusty? Hasn't all the dust blown off by now?
My question is: Why do comets appear to have a continuous supply of dust?