I can never quite remember the sources of my info. I believe, but don't quote me on this, that figure was from Johnathan Haidt... I believe you can find it in the book "The Happiness Hypothesis".
Maybe it was another cognitive neuroscience book. Regardless, 3-4 years is the number I've heard been tossed around.
Regardless of the length of time, point holds true.
Passion is a self generated drug in the body, not dissimilar to adrenaline that actively causes behaviourial changes in the people experiencing it.
The amount and the longevity of this passion drug varies from person to person - meaning that any two person pairing is likely to have some degree of divergence...
And that once adjusted for, if the fundamentals of a relationship... communication, cooperation, companionship... are weak, then it'll result in cheating or an ending of the relationship.
This natural ebb and flow is compounded for by the fact that as a culture, we popularize the notion of 'one true loves', and forever passion and all kinds of rubbishy junk that leads us to believe that we're broken inside when shit doesn't pan out (or rather when shit pans out exactly how it should pan out).