I have been getting about 3 winter seasons out of a 20" Marathon Winter on the rear wheel of my folding bike, and about 4 to 5 winters on the front. I'm in Chicago, where there is a lot of salt on the streets. After a while, the little steel "hats" that hold the tungsten carbide studs in the tire begin to rust. The corrosion products act as an abrasive, and the stud hats eventually wear through the tire carcass, where they cause flats. I also typically lose 2 to 3 studs each winter (on the rear only); I don't replace them because the rubber is distorted. The tires work fine since there are plenty of studs left.

I don't know to what extent the 20" wheels cause the studs to be lost or corroded. I suspect the rear tire takes more of a beating because it bears more weight and delivers all the massive power to the street.
The problem of flats is completely cured by using a tire liner. I took the liner out of the rear tire I had used for three winters and saw stud bases clearly visible, yet I had no flats.

I've installed a new Marathon Winter on the rear without a tire liner, and a front that has been used through two winters, with a tire liner even though no stud wear is apparent. Just to be safe!
Steve

EDIT: Images of the inside of the rear tire, and the outside of the same section of the tire.