In fact, since 1995, only Dennis Erickson with Seattle and Bruce Coslet with Cincy, have been allowed to coach for 4 seasons without making the playoffs.
As an exception to the three year rule, starting with Mike Shanahan in 1989, the Raiders have dismissed six of their previous eight coaches after two seasons or less.
To see how often team's actually have a successful coaching hire let's analyze the tenure length of NFL coaches of ten years ago, in 1997.
Gone after 1 season:
Joe Bugel- Raiders
Gone after 1.5 seasons:
Kevin Gilbride- Chargers
Gone after 2 seasons:
Lindy Infante- Colts
Gone after 3 seasons:
Ted Marchibroda- Ravens
Pete Carroll- Patriots
Mike Ditka- Saints
Bill Parcells- Jets
Dick Vermeil- Rams (Won Super Bowl)
Gone after 4 seasons:
Vince Tobin- Cardinals
Dom Capers- Panthers
Bruce Coslet- Bengals
Barry Switzer- Cowboys (Won Super Bowl)
Bobby Ross- Lions
Jimmy Johnson- Dolphins
Ray Rhodes- Eagles
Dennis Erickson- Seahawks (coached 6 years total- never made playoffs)
Gone after 6 seasons:
Dave Wannstedt- Bears
Steve Mariucci- 49ers
Tony Dungy- Bucs
Gone after 7 seasons:
Dan Reeves- Falcons (reached Super Bowl)
Mike Holmgren- Packers (Won Super Bowl)
Norv Turner- Redskins
Gone after 8 seasons:
Tom Coughlin- Jaguars
Gone after 10 years:
Marty Schotteheimer- Chiefs
Dennis Green- Vikings
Stepped down after 12 years:
Marv Levy- Bills (made 4 Super Bowls)
Stepped down after 15 years:
Bill Cowher- (Won Super Bowl)
Still with their team:
Jeff Fisher- Titans (14th season- Made a Super Bowl)
Mike Shanahan- Broncos (13th Season- Won 2 Super Bowls)
None of the seven coaches who failed to make the playoffs lasted more than four seasons, one coach who won the Super Bowl last only four, and another who made the playoffs twice lasted only three.
Knowing this, let's see who is on the hot seat after this season?
Brian Billick is in his 9th season with the Ravens but a late-season swoon might cost him his job.
Prediction: One more year
John Fox is in his 6th season with Carolina and made a Super Bowl in 2003, but his team is falling apart, and the only thing that might save him is that he has a built in excuse with the worst QB situation in perhaps the history of the league.
Prediction: Fired, though he will be swooped up quickly
Mike Nolan is in his third year with the 49ers and hasn't smelled the postseason, so keeping him would break the 3-year rule. Plus, he has bickered with his (possibly inept) young QB. His only defense is that he might have the least talented offense in the league.
Prediction: Either Alex Smith goes, or (more likely) he does
Joe Gibbs is in his 4th year, finds himself in a brutal division with a soft team.
Prediction: Gibbs is allowed to "retire" gracefully.