Hmm. We have 2 sets of wheels, one for summer with dedicated summer tires and one for winter, with dedicated winter tires.
My calculations:
When driving on whatever set, the other set is NOT used. Therefore, assuming that with my summer tires I can drive 40k miles during 20 months (just an example), and that during any year I use them for 6 months /y, I can drive my summer tires for "3 years and 4 months". Similarly, for my winter tires, I can drive maybe the same amount of miles during 20 frosty months (just another example). During any year, I would use them for 6 months (October, November, December, January, February, March), so I can drive them also for more than 3 years. Therefore, after 40 months driving, I have to change 2 sets of tires.
Now, the problem starts when driving Winter tires during Summer months. They will wear much faster, but I never did a controlled experiment to measure just how much faster. Let's assume 2 times faster. That means that you can still drive them during the 6 frosty months for a total of 20 frosty months. However, some of these months are warm (let's assume half of them), so out of the life of 20 months, you will loose 6 or more, because of the warmth. Hence, you will have to change your tires after 14 or 15 months driving. Aka, after 30 months driving, you would have changed 2 sets of tires.
Of course, this is just my line of reasoning. The other is that my wife commented that with winter tires in the snow, the car reacts much better than with summer/all season tires. She drives our Volvo XC90 like a Cooper Mini S, so she knows what she is talking about. And this is apart from the safety aspect. I have 4 kids, and I am willing to invest in whatever safety I can buy for a reasonable amount of money (winter tires, child seats, etc).
KR,
Henk