Well it depends on the weather - this year, autumn has turned out to be cool and damp, but October can often be dry, sunny and fairly windy, so the usual time to turn it off would be end of October in London and the South East, but probably earlier in the west and north of the country. This assumes your automatic system is watering the ground, not pots.

mine is in the greenhouse and it gets turned off when the toms are about over . as they dry out they get a watering can full when needed.