I had this exact question last winter. I bike all year round and switch the tires once per season.

The advice I got was essentially, keep them on because there's lots of winter left and it's cheaper to keep them on than risk a wipeout and possible medical bill in case winter makes a sudden return.

The advice I got was keep them on for about two or three weeks of low temperatures above 32F. If it can't freeze then no black ice thus time to start thinking ahead to spring and summer.

For me this means about the Master's PGA tournament, which is April 11-14 this year, or one week before Easter