Tires don't last for ever, even good ones. I think it depends more on where you ride than anything else.

I've just replaced the rear tire on my bike again. This was a Marathon that was new in September, and it lasted 1100 miles. I've had about five flats in that time. Riding home on Friday it developed a bulge that proved to be an inch long gash along the edge of the tread. I rode it for a while, but around a mile from home it was pushing out a big bubble of my slime liner, and I thought it might blow any minute, so I let out the air and walked the rest of the way. There were three other gashes in the tread of the tire, but these didn't go through the casing. It's predecessor, also a Marathon, had lasted only since July. Before that, I had gone through two Primo Comets. That's four tires in 15 months, averaging less than 1000 miles per tire.

My new tire is a Maxxis Hookworm. Wish it luck!

Edit, forgot to mention, this was on my commuter bike. I have better luck touring!