Motivated by looking at famous coaches who lasted a long time with one team and were not ultimately that successful, I checked the record of the guy who is the closest comparison. It turns out Lewis has a lot of work to do just to get up to that level.
Jim Mora, New Orleans Saints, First 9 Years:
Record Overall: 84-63 (including four playoff losses), 57% winning percentage
Record Against Playoff Teams (including playoff games); 21-39, 35% winning percentage
Average Score when playing Playoff Teams: 17.4 to 20.7, losing by 3.3 points
Marvin Lewis, Bengals, First 9 Years:
Record Overall: 69-77-1 (including three playoff losses), 47% winning percentage
Record Against Playoff teams (including playoff games): 14-46-1, 24% winning percentage
Average Score when playing Playoff Teams: 18.2 to 25.1, losing by 6.9 points
Jim Mora Playoffs ! ? Long Version (via gordievsky)