At work, we discussed when to change to winter tires for bicycles (and cars). We assumed that temperature was the key factor and I used a program (Mathematica) to check the mean temperature in Stockholm (see this question Probability of ice in Stockholm).
My question is: besides the risk of ice, what are key factors for deciding when to change from summer tires to winter tires? E.g. is the rubber of summer tires negatively impacted when it gets colder.
I weigh the advantage of increased control against the cost of snow tires.
Studded tires take quite a bit of wear on dry pavement. Each stud type will have a different life span based on materials tire shape etc. My anecdotal experience shows stud life on dry pavement to be about 500 miles. I have done winters without studs because it never reached a consistently icy state. The handful of icy days could be mitigated by route choice (both to avoid dangerous traffic locations in ice, but also to avoid shady spots where ice is more likely).
I switch over once it starts reaching 3 icy days per week or a commute route which has over a 1/4 mile of constant snow/ice. My winter commuter is a rigid frame/fork mountain bike with relatively fat tires (~1.75 in) at low pressures (25-30 PSI). It is quite stable and a major contributing factor in holding off on studs until it gets very icy. That said I took 2 falls last winter without studs due to ice. No other falls in the past year.
The cost of burning through studded tires can start to add up. I find it more efficient to keep a pair of studded tires installed and ready to go on an older pair of wheels, and I can just throw them on if it looks like it's going to be an icy day.
The first day of Winter (or winter solstice) in the northern hemisphere is usually December 21 or 22 every year. However, depending on the conditions, you may want to change over to Winter tires well before that.
Depending on the year, a slurry of other factors, and your location you may get snow and ice well before then. You should make an earnest attempt to have changed your tires at some point before you begin to see snow and ice material forming in your area.