Last July I predicted that The Dark Knight would drop from #1 on imdb to somewhere between #s 15 and 22, although I conveniently didn't mention a specific time scale. Nevertheless, a year or so is a reasonable amount of time to consider, and since I am sometimes a reasonable person I will admit that yes, I was off. It remains at #7 today. Here are some charts with data from its relatively small decline over the past year. Most of my data points are from the first few weeks, because that was when it was most exciting.
I'm missing a bunch of data points past the first few weeks but you can see in this last one that the chart fits a power function well. The decay will stabilize at equilibrium at some point--I wouldn't expect the movie's rating to drop foreover.
The Dark Knight has set the bar for imdb chart domination in the modern era. Despite its impressive staying power, it, like every movie before it and like every movie after it, has dropped. This is the rule for imdb ratings. The question is whether the imdb rater's tendency to favor new stuff is specific to the movie industry or whether it generalizes to other domains as well. Too bad there are so few quantifiable rating systems.