How long does an alternator last?

Ok I asked this question earlier and only got one lame response about a conspiracy theory. Let me saw these are the facts, and everyone already knows that there is much controversy over the intergrity of some ppl who work in the automotive field. So I took my vehicle to my uncles garage yesterday to get my tail light changed. I left it there and picked it up after hours. Except when I went to start it, it would click, but never turned over. We jumped it off and drove home. Now it wouldn't start this morning. I am thinking it is my alternator. Which brings me to the question of how long they usually last? I have been through 3 different alternators in 4 years. And 2 were on a different vehicle. So maybe I wouldn't have thought twice about it except, that I have heard my uncle bragging about over charging someone for labor they didn't need. Plus they have recently went throught legal issues. Also, everytime I go there for an oil change or maintenance, something else is wrong like a sensor, or my ac. So that brings another question, is there any way to tell if my car (alternator) has been tampered with?