Originally Posted by sweeks

I believe the wear is due to abrasive corrosion products (rust) formed from the steel "hats" that hold the carbide studs in the tire. Corrosion is accelerated by salt used to melt snow, so if your riding is more on un-salted trails than streets, your tires may last a much longer time.
Another sign that the studs are wearing the tire is when the studs are getting "cock-eyed" in the tire. Here's an example:


This Marathon Winter has seen two winters on salted Chicago streets. Missing, corroded and cock-eyed studs are signs that the inside of the tire should be inspected. This tire was on the rear wheel, which carries more weight and (of course!) has to transmit the rider's enormous power. LoL!