On the Ford Taurus with the 3.8 V-6 engine the serpentine belt lasted me around 50K miles before it needed to be replaced. There were no signs of ripping or tearing on the old belt just some squealing when the A/C was turned on. The squealing comes from the belt tensioner being unable to apply enough pressure to keep the belt from slipping. When the belt gets stretched and dried out the tensioner can no longer hold enough pressure on the belt. You just need to replace it.
To replace the serpentine belt you need to relax the tension on the old belt before you can remove it. Before you remove the old belt it would be a good idea to make a drawing, or schematic of the pulleys and the direction the belt takes so it will be easier to install the new belt.
To relax the belt tensioner you will need a half-inch drive breaker bar. On the pulley arm there is a half-inch square hole that the breaker bar end needs to fit into so pressure can be applied to back the pulley off of the belt. Clearance is very tight in that area and sometimes a breaker bar won't fit. On my Ford it was so tight getting to the belt tensioner I had to go to the hardware store and buy a one inch piece of square stock. I took the square piece of stock and stuck it into the tensioner and used a wrench to move it.
That worked out well, I maneuvered the wrench so I could work on moving the old belt off of the pulleys then put the new belt on. With the tensioner still relaxed I slid the new belt on to the pulleys, once the belt was on and seated I started the car and checked the belt to make sure that it was tracking on the pulleys right. It is good the check that one side of the belt isn't out of its pulley because that will ruin the new belt quickly.
