Does anyone know how many miles per year the average driver drives?

One driver, 45k-50k. Two drivers, about 80k/yr.

How often do you change your tires?

Depends on how good they were to start.

How do you decide which tires to buy and from whom?

Depends if you have a flat and need it right away, you take what you can get at a roadside shop, used. If you are going for an inspection, you might want to go with new tires.