Mine is 10 years. I experience a longer than usual honeymoon phase (between 3 & 5 years). Then I start to lose the urge to initiate sex. Then get into a sexual rut. Start over focusing on work (I think as a distraction). It's all downhill from there really. By the time I hit 10 years in the relationship, I should really walk away. I never do. Should know better really. 10 years is all I can manage.
It's always me that has to end the relationship though. No matter how shit it's become.