If you want to get rid of it, anytime. If you are "cutting it back", it depends on where you are. If there are signs of new growth, it's too late. Otherwise pick a time when the grass is dry enough to burn without help. Better sooner than later.

Please remember that inside the heart of the pampas grass there will be overwintering beneficial insects like ladybirds, lacewing, and even small mammals. Setting fire to it in the spring before these emerge will be killing them.
In nature, pampas grass would be burned naturally in dry periods and heatwaves, so burning in summer might well be better as the roots will regenerate the plant if you want to keep it.
If you want to destroy the plant completely, you can only really do so by resorting to the spade and pickaxe, or lots of noxious chemicals.