As soon as the weather forecast says there will be snow and it's going to stay. I personally don't care about the 7 °C rule that summer tires all out of a sudden lose friction once the temperature is below 7 °C, but that's a whole different discussion.
I guess it all depends on where you live. If the terrain here wasn't as flat as it is, I would probably switch sooner. Or if I depended on my car for work. Or if I had to ask someone to change the tires for me (I use an extra set of wheels with winter tires). If you call your local tire shop the day after it starts snowing, you might have a problem. In either of those cases it would make a lot of sense to switch sooner rather than later.