Let's take a look at the 2011-12 season (the fifth overall) of Kitchen Nightmares on Fox.
Rating the Ratings: For most of its 17-episode season, Kitchen Nightmares settled in as one of the strongest players on Friday night and kept Fox in the Friday game. It won its 8:00 timeslot nearly every week, though you can argue that beating up on A Gifted Man, Chuck, and a fast-crumbling Extreme Makeover: Home Edition isn't that much of an accomplishment. Things soured somewhat in Nightmares' last few episodes as the competition ramped up. CBS had a breakout performance from Undercover Boss and ABC also considerably upgraded with the return of Shark Tank. But the show's season finale was on March 30, so its originals were mostly able to avoid that morass. It's getting held for midseason in 2012-13 as Fox tries to get something going with sophomore drama Touch. With Fringe ending in the fall and Touch a question mark at best, I'd expect Nightmares will be back in the Friday mix again by season's end. Grade: B-.
Here's the now updated War of 18-49 post for Kitchen Nightmares.