These belts are very long lived. Degradation is a function of time at temperature, so lots of low speed traffic in hot weather probably means less life than highway speed in cool weather.

The maintenance schedule for my car calls for an inspection at the 30K maintenance interval, but no specified change interval. It's good until it fails inspection. I ordered the belt replaced just before 50K miles circa 1992 when the water pump was replaced on warranty (only cost me the part), and I'm still on that replacement belt, which still looks new.

During my annual belt inspection I also try to wiggle all the pulleys to check the various bearings because if a pulley picks up a wobble or misalignment due to a failing bearing, it could take out the belt. On M103s the belt tensioner pulley is known to fail and take out the belt. One friend had a belt tensioner pulley fail, which took out the belt and the belt took out either the power steering or auto trans cooling lines, which made a huge mess including taking out the hood pad with a combination of belt slapping and oil saturation.

It pays to inspect the belt and pulleys frequently!

I may just go ahead and replace the belt apriori at 90K if I haven't changed it before then. At my current rate of mileage accumulation it will take me between five and ten years to hit 90K.

Duke