There's no set guideline. You just have to eyeball it, by looking at plant health and day & night temperatures. Plants tolerate cold nights better if the days are warm. If the days are cold (single digits) and dark, the plants will quickly deteriorate. Also, if you use row covers, you can add a few additional weeks or more of a harvest window.

In general, don't leave root crops in after the soil begins to freeze (usually around Halloween in SK but this varies from year to year). However, carrots and beets tolerate this better than radishes and turnips.
Some leaf crops are very hardy, things like kale and spinach can take -12 C before they begin to show damage. Cabbage is a little less hardy. I haven't grown brussel sprouts but I believe they should be good down to -10 C as long as the plant has had a few nights to acclimate. Again, watch the daytime temperatures.
In most cases, the first lasting snowfall in early November is the time you pick everything. You can still leave kale in the ground but it's too much a bother to try to harvest it under the snow.
I use row covers to prolong the harvest to Remembrance Day most years (last year was an exception). I live near Saskatoon.

Avoid harvesting anything while it is still frozen. Wait until late afternoon to let everything thaw out and perk up before you harvest.

This post was edited by Slimy_Okra on Mon, Sep 23, 13 at 0:21