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.