It's probably easier to tell someone which carrier will be the best one for them than it is to put a time or mile lifespan on an alternator. There are too many variables.The rotor is spun on a needle bearing, which can break due to excessive wear or dirt and grime that can build up inside, the diodes inside the voltage regulator will fail after a time from the heat from the engine putting an extra strain on the part.
It depends on the vehicle, the engine, the conditions in which it's used, how much electrical equipment it's regularly operating and so on. You are giving it more of a workout than you realize, though, even without using an inverter or coffee maker. The on board computer and sensors are using quite a bit. And driving down the road with headlights and air conditioner on may be close to maxing it out.
Some cars may lose an alternator at 40,000 miles while others may continue to operate well after 100,000 miles. Medium duty vehicles, like F- and E-350s have a heavier duty alternator that should last 200,000 to 250,000 miles, generally speaking. That doesn't mean they won't fail at 75,000 or last for 300,000.
They do get weaker over time, though, as the diodes and brushes wear. With a cold engine, start it up and use a voltmeter to measure the charging voltage at the battery terminals. It should be 14.0 volts, give or take a point. (If it's a hot day and the engine is already at operating temperature, the voltage will be less, like 13.8 or less.) Now start adding things like headlights, then radio and high fan speed and air conditioning. If the voltage hangs tough at the batteries the alternator is fine. If it drops significantly, then the alternator is on its way out.
At 200,000 is getting close and it's good to be thinking of a preemptive replacement. I replaced mine at 250,000 miles and carried the old one as an emergency replacement. The new one at 120,000 miles threw the decoupler pulley so I'm glad I had the spare handy. The spare only lasted about 50,000 and I replaced it. 240,000 miles later I replaced it again because it was getting weak, what with time, hours used, oil and grime all over it.
So I think replacing yours soon is a good idea. Either keep the old one as a spare, or have it rebuilt and keep as a spare. I would recommend replacing it with OEM or a very high quality aftermarket (like Denson, who is also an OEM for many auto makers). Stay away from the cheap ones like, say, from from Autozone, as the alternator is a critical part for what we do.
