The No. 1 reason a alternator goes bad is the brushes wear out. These cost about $0.25 each.
The No. 2 reason is the voltage regulator goes bad. Unlike No. 1, this greatly depends on the make of car you have. That is, it depends on the design. If the voltage regulator gets hot, it can fail sooner. Some designs run hotter than others.
Rebuilt alternators are often junk. The big rebuilding houses buy old alternators by the truckload. They separate them by make, then disassemble them, throwing the housings in one pile, the rotors in another, and the stators in a third. Then they clean them and reassemble. But, they aren’t careful to make sure they don’t have one half of the housing from a 5 year old one, the other half from a 10 year old one, a rotor from a 4 year old one and a stator from a 12 year old one.
Why does this matter? It has to do with tolerances. All of these parts are made to meet specified tolerances. But, on the new assembly line, if the housings coming from the die-casting machine are to the high side of the tolerance, they may run bearings towards the low end of the tolerance range to balance things out (or similar). That is, they can easily afford to match tolerances to have a better fitting component. They do this to make their product last longer and gain a reputation of high quality.
As I mentioned, on rebuilt units, they aren’t as careful. It generally doesn’t matter, since once a car has worn out the original alternator, it is probably near dead so the replacement alternator only has to last a few years, not a decade or more.
If you know you have the original alternator (or starter motor) for the vehicle, you will be better off having it rebuilt than buying a rebuilt one, since they aren’t likely to swap parts if you take it to a local shop for a rebuild.
Oh, one thing that will kill a brand new alternator is to install it in a vehicle with a partially charged battery. With new brushes, there can be quite a bit of arcing if the current load is high (it gets less once the brushes are worn in). This arcing can cause pitting on the contacts, causing the brushes to wear very rapidly. The high initial current load can also blow the voltage regulator. Always make sure you have a fully charged battery when you replace an alternator. New batteries are generally only about 3/4 charged, so it is a good idea to put it on a battery charger. A “good” mechanic will do this when he replaces an alternator. Some may not.