Depends on where you live, I generally switch them in end of November and take them off middle of March.
Every year is different, but a good estimate is to check a frost chart for your location and to decide what level of frost is going to be hazardous for your type of riding:
I tend to cheat the 90th percentile 28 degrees F dates by a week or two, but again, it varies each year, depending on what's happening locally with the weather.
In my opinion, when to take them off is a much more difficult decision than when to put them on, because the warming days of spring are much more hazardous due to daytime thawing with melt waters flowing and then freezing back onto the hard surfaces just as the sun drops off = higher risk of ice for evening commuters + at the same time, you're just sick of riding on the darned things by then and can't wait to get them off. If you persist with winter riding, you'll likely wind up with a second set of wheels to take advantage of periods of warmer weather.