Jim Mora (the elder) resigned from the Saints midway through the 1996 season after 10 1/2 years as by far the most successful coach in the team’s 38-season history.

“I probably should have gone before that,” Mora said a couple of years later, after taking over the Colts. “In this league, eight years is about the longest a coach should stay with one team.”

Make Bill Cowher the exception, make Mora’s dictum the rule, and apply it to the likes of Mike Holmgren, Mike Shanahan and Mike Martz, all of whom have been to the Super Bowl but are struggling this season.

Yes, Holmgren and Martz have less than eight seasons with their teams and, yes, all three could still make the playoffs, even with teams that have been major disappointments.

But change is good — although Bill Parcells has changed teams and done well, but never as well as he managed previously.

Look at two current coaches, prime examples of guys who were fired despite success:

• Tony Dungy, who took over in Tampa Bay in 1996, turned the Bucs from a laughingstock to a playoff team and was fired after six seasons because he couldn’t get them to the Super Bowl. He was hired by the Colts less than two weeks later and is 35-15 with two games to go in his third season in Indianapolis.
• Dennis Green got to the playoffs in eight of his 10 seasons with the Vikings before he outlived his welcome with owner Red McCombs. Now in his first year in Arizona, he is making one of the NFL’s historically dismal franchises respectable, although he may have blown his best shot of winning in that weak NFC West by playing musical quarterbacks with Josh McCown, Shaun King and John Navarre during a three-game midseason stretch.

This is not to say that Shanahan, Holmgren and Martz all will be fired the way Dungy and Green eventually were.

Shanahan has a vote of confidence from owner Pat Bowlen — the legitimate kind, not the kind that leads to a firing a few weeks later.

Holmgren and Martz may be on shakier ground, although Holmgren would surely end up with another job. He comes from the San Francisco area, made his name as an assistant there and the only thing that would prevent him from going back would be the frugal tendencies of owner John York.

Some stats:

Shanahan has coached the Broncos since 1995, has won two Super Bowls and has a record of 106-62, a resume that needs no defense. But since John Elway retired after the second Super Bowl win following the 1998 season, Denver is 52-42 and has not won an AFC West title or a playoff game. That mark is OK for many teams, but not for the Broncos and their fans.

The quarterback factor is major in Denver.

And in Seattle.

Just as Jake Plummer has not been Elway, neither have Jon Kitna nor Matt Hasselbeck been Brett Favre.

Holmgren and Favre both arrived in Green Bay in 1992 and Holmgren was 84-42 there, winning one Super Bowl and getting to another, where he lost to Shanahan and Elway. In Seattle, where his primary QBs have been Kitna and Hasselbeck, Holmgren is 49-49 and has lost two playoff games.

Martz is a little different.

He was Dick Vermeil’s offensive coordinator when the Rams won the Super Bowl after the 1999 season, then became head coach when Vermeil left after that game. He got the Rams back to the Super Bowl two years later, but was badly outcoached by Bill Belichick, losing 20-17 in a game in which St. Louis was a 14-point favorite.

Martz is acknowledged as a brilliant offensive mind, but his game management is often quirky, as it was in that Super Bowl, when Belichick often used six and seven defensive backs, but Martz declined to pound Marshall Faulk at the undermanned front.

He also wastes timeouts and challenges because he believes his offense can overcome any obstacles.

So in a 2001 game with Tampa Bay, he was out of challenges when Warrick Dunn clearly stepped out of bounds 8 yards short of the end zone on what turned out to be the winning score. That was after Martz had used replay to get the ball moved back a few yards on a meaningless kickoff return.

Martz’s status is up in the air. Shanahan won’t be fired and if Holmgren goes, it is likely to be voluntarily.

The Seahawks were probably overestimated in the preseason, when they were thought to be the second-best team in the NFC behind Philadelphia. They should make the playoffs, and as Holmgren said this week, his voice trailing off: “once in the playoffs …”

Finally, there’s the Cowher example.

He became coach of the Steelers in 1992, got them to the Super Bowl after the 1995 season, but has had a run of disappointments, losing AFC title games at home to San Diego, Denver and New England, plus a couple of 6-10 seasons, including last year. But Dan Rooney, the team’s owner, is a firm believer in coaching continuity and has declined to bow to the occasional fan and media pressure.

This year, Cowher has rewarded Steelers fans with a 13-1 record, going back to depending on the running game that has served him so well in the past. Some of that may be fortuitous — when Tommy Maddox was injured in the team’s only loss, in Baltimore in the second game, Cowher was forced to go with rookie Ben Roethlisberger at quarterback and play more conservatively.

Cowher is an exception. Mora the elder’s eight-year rule is probably a better barometer.

Just ask Tony Dungy and Dennis Green.