Although we have a two belt system, it is still considered a serpentine belt. I don't have a 99 either, but I thought I'd share a few things that have run through my head concerning the belts. I have an 02 and although my belts have lasted more than a year, I have experienced a failure. The belt for the water pump and generator did break on me. The belt for the A/C compressor and powersteering has six ribs, whereas the waterpump/generator belt only has four. The smaller belt broke on me at 45,000 miles. When I replaced them, I found very little wear on the larger belt.
Recently, I replaced my motor and found that the smaller belt was in need of replacement and once again, the larger belt was in great condition. At that point, I had put another 75k miles on the motor. The replacement belt went 30k more miles than the OEM belt and was still intact. I feel the Gates belt that I installed after the first failure was well worth the extra couple of dollars than the other options offered at the parts store.
There are pretty big differences in belt manufactures. I've had good luck with Gates and Goodyear. Since this has been a reoccurring problem for you, I'd get in the habit of inspecting weekly. Another factor to consider is belt tension. Is it possible that you overtightened? Did you test belt deflection after your last replacement? Also, check for proper pulley alignment and inspect the pullies for irregularities.
