Pew Research Center on the divorce rates for long durational marriages

THE PEW RESEARCH CENTER ON LONG TERM MARRIAGES and THEIR DIVORCE RATES
The Pew Research Center has done a number of studies on divorce. One of the most recent is a study on long term marriages and the likelihood of these marriages ending in divorce. Says Pew:

On an annual basis, the probability is very small, according to Census Bureau statistics from the American Community Survey. In 2008, among couples married for four decades or more, half a percent or less had obtained a divorce in the previous 12 months. (Among couples married for 25 years or more, the share that divorced in the previous year rises to about 1 %.)

It appears that the longer a couple remains married, the less the risk of divorce. In other words, as the number of years of marriage increase, the rate of divorce decreases. So there appears to be an inverse relationship between duration of marriage and rate of divorce. Here’s Pew again, qualifying the above:

What these overall statistics don’t say is that the risk of divorce is not the same for all groups. Adults with a high school education or less are more likely to divorce than are college-educated adults. People who marry young are more likely to divorce than those who marry at older ages. There also is some early evidence that couples who married in the 1970s may be especially at risk of divorce. According to data from the 2004 SIPP, the share of marriages that ended before their 15th anniversaries (mainly because of divorce, but also in a small number of cases because of widowhood) was lowest for marriages made in the 1950s, followed by those made in the 1960s and then those made in the 1980s. Marriages made in the 1970s were slightly less likely to reach their 15th anniversary than those begun in the 1980s.

So, it’s not just a matter of how long a couple remains married. Other factors such as the age they were when they married, the decade in which they married, and even their educational background could be mitigating. But it appears that the gist of Pew’s study is that, all things held equal, the longer a couple remains married, the lower their risk of divorce becomes.
Read more here.