When should you stop feeding hummingbirds?

Never, they need twice their body weight in nectar to survive. If not, they will die.

-------------------------------------------------

If you live in an area where hummingbirds migrate south for the winter you should keep the feeders out until nectar is no longer being taken and make sure that the feeders are out early in the spring to catch the first arrivals for the next summer.... basically the first answer is correct if you live in a country where humming birds are present all year.