I’m not sure about my parents. Maybe 20 years maybe a bit more. Couldn’t tell you when they met or married or when they broke up either exactly, I don’t know which affair you would count as the one to break them up or is it official date of moving to live apart or the court cases or the actual divorce date which is many years later? Can’t ask them as they are dead.
My longest relationship is my current one at 24 years. No plans to marry or have a messy divorce.