It's not the marital status that determines how long a couple stays together.
As long as two people want the same things and agree on how to have them then they will stay together.
You also have to define what marriage is to you. If it simply means (a couple has not filed for divorce) then that would include couples who have not lived together for years. It also would include those living separate lives under the same roof. She lives upstairs and he lives in the basement. Bacially they're roommates with the same last name.
I know of one such couple where the wife resides in their home in Chicago, IL and he stays at their home in Lake Geneva, WI. They attend family functions together, discuss bills, and go to each other company's events. This has been going on for 13 years. "Technically" they've been married for 35 years.