The duration of a "culture" is really quite variable, but 100 years is actually a fairly long time for some of the notable recent ones. The real point though is not how long the civilization lasts, but how long the gap between two civilizations may have been.
Many cultures that last for fairly long times can be pretty much destroyed and disappear during a rather brief time, only sometimes due to invasion or natural catastrophe. An existing culture that's been reasonably stable for a long time, but that hasn't had much influence on anyone else can "rise to influence" within the period of one conquerer's active military career, i.e. within 20 years or less. The starting and ending points of many cultures are "abrupt," so far as their impact on others, or their influence over wide areas, are concerned.
Minor things like plague, typhus, typhoid; a blight in the corn crop - or the potatoes; a "benign" visitor with the measles or a "righteous" one with a bonfire; or the trivial change in political/religious leaders and changes in rites and practices; a single ambitious politician or general; a select assassination or two; or even something as trivial as a little volcano, can cause abrupt passages from one culture to a new one with a distinctive and different personality.