[masraum]

They can last a very long time. Lots of things will affect longevity. Living in a very hot environment will often reduce the life of electrical components. A stuffed engine bay that allows very little air circulation is going to be rough on everything under the hood. Something with a bunch of room and air under the hood will be better. How the vehicle is driven (hard, easy, etc...)

It seems like these days, many of this kind of part can last a long time. In alternators, it's often one or two small parts that will require replacement, but it's usually more common to just replace the whole thing.