Leave them up as long as you want. Birds don't read books telling then when and where to migrate. Just because somebody else believes hummingbird feeders should be taken down earlier doesn't mean you have to, there your feeders, do what you like. Anna's Hummingbirds seem to like to wander in the fall and they may show up late just about anywhere these days. If there are no flowers in bloom, they need a source of food.
Last year I was too lazy to take my feeders down and we got an Anna's Humming bird on November 19th, and it was a good thing I still had the feeders up. It was starving. I stood 3 feet away from it and watched it feed for about a half hour. (I was getting ready to put fresh food in the feeder shortly after my son spotted the Anna's near the feeder few minutes earlier. As I was reaching up for the feeder, the Anna's came in and started to feed with me right there. It was fun to watch it up that close.)
Obviously, if your hummingbird food is frozen solid, it's probably time to bring the feeders in. (Our Anna's stay until December 9th. I would bring the feeder in at night and put it back out early, before sunrise. I also made a heated enclosure for it to.) Other people have had hummingbirds later than December.