This depends on your location, and how deep the ice tends to grow into the water. This also depends on what kind of fish you choose, and how much room they need to thrive. The rate in which ice grows depends on the ambient temperature, the wind, snow cover, and how thick the ice is currently. Thick ice grows more slowly than thin layers of ice, and a cover of snow delays the process further. Most fish require about 2 or 3 feet of water to move around in, even in their winter dormancy. Make sure your pond depth takes into account how much ice you expect to see, and leaves 2 or 3 feet underneath for your fish to live.
In order to encourage your fish to enter their dormant period, you'll want to reduce the frequency of feedings as it grows closer to wintertime. Purchase a pond thermometer and keep an eye on your pond's temperature. Once it reaches about 50 degrees, start spacing out your feedings further away from one another. This sets off their instincts to begin preparing for winter, and their metabolism will begin to slow in response.
