It is critical to your bees survival that they go into the winter with adequate food supplies. Begin feeding as soon as you pull the supers off and continue until the bees are no longer drawing it down. As it gets colder, the bees will be less interested in feeding. They will stay balled up and not go up to the feeder. Other times they will fill up the space they have as they dehydrate the syrup down to the moisture content of honey. In Wisconsin it is best if you have in excess of 50 pounds of honey in the hive.
The syrup you will feed the bees in the fall is a more concentrated sugar than the springtime syrup. Mix it by using two pounds of cane sugar and one pound of water. This will be harder to fully dissolve but will more closely mimic the full-time nectar coming in from plants. This thicker syrup keeps the queen in the mode of not laying eggs.