It lasts as long as your steering components last. Once you get one of them changed or the are worn out from use you need to go in for an alignment, after getting the parts replaced in almost 99% of all cases.
Things that mostly cause a car to be or seem to be out of an alignment are bad tires, under or over inflated tires, non rotated tires (ones that been on the front for a very long time), road issues (not all roads are made the same), and worn front end parts (tie rods, worn rack and pinions, ball joints, and strut bushings on newer cars, center links, idler arms, pitman arms and ball joints on older cars.). Oh and poorly repaired cars after a wreck. I have seen cars come in that were so poorly repaired that they couldnt even be aligned properly because the unibody or frame was so out of whack no adjustment to the max could correct it.