I've just finished reading Roger Zelazny's Amber series of books and two things have come to mind while I've been reading. One is that J. Michael Straczynski seemingly stole the core concept of the novels for his television series Babylon 5 from this series. The other is that books have gotten a lot longer and movies have gotten shorter since Mr. Zelazny wrote his series of Amber novels.
The way in which movies are edited to run under two hours, with a seemingly preferable running time of 90 minutes, has always annoyed me. There was a time when movies were so lengthy that intermissions were scheduled between a first and second segment while reels were changed. I'm pretty sure that the longer running time for movies in the past was one of the reasons concessions stands were in theaters. If you're there for 3 hours, a pit stop in the middle where you pick up a drink and a snack makes more sense than if you're only around for an hour and a half.
I have read that the nadir of short running times for major movies was marked by Men In Black II at 88 minutes. I believe a minor reason for all the short running times is that movies are now demographically targeted at young males who, supposedly, prefer movies that are short and fast-paced. In the 30s, 40s, and 50s, movies tended to be targeted more toward women. This was back when the audience was determined by who attended most frequently and, at that time, women were more likely to go to movies repeatedly.
Of course, there's a real "chicken and the egg" quality to who is attending movies more often. If you make short action movies which are big on special effects and light on story, more males will go than females. I have to imagine that the audience you cater to (or pander to, if you wish) is the one you're going to attract.
The larger reason movies are so short these days is that it's more profitable for theaters to have short running movies because they can squeeze in more viewings each day. Fewer screens will show a film if its running time is too long and its not part of a proven sequence of blockbuster releases (such as "Lord of the Rings").
In terms of books, I have noted that a lot of what would be considered "pulpier" entertainment has been growing to relatively gargantuan sizes. The Harry Potter, series, for instance, seems to get longer and longer with each new release. I wish I could say that the books getting longer means that we are reading more or more willing to read a denser story but the truth is that it's more of a reflection of the power an author wields once she (or he) has achieved a huge amount of success. J.K. Rowling in particular is not a better author for providing more content. She seriously needs to have her content edited in her last several books but she's powerful enough now to have it her way and there is every likelihood that the fan base, which likely isn't able to scrutinize the quality of a book based on the presentation of the story because they are simply enamored of the characters no matter what they're doing, are happy to get quantity over quality.
Similarly, other highly successful authors can either write huge books or string out one story for a series of books in a manner that less successful authors cannot. Stephen King's "Dark Tower" series is a good example of such a thing. The story went on and on and on and some of the books were rather spotty in terms of the value of their stories in the overall arc. The only reason the series was completed (or possibly the only reason it wasn't a trilogy instead of spanning 7 novels) is that Stephen King was the author. While I liked the series, I thought it could use a good prune and would have been much better for eliminating most of some of the content. For what its worth, I think Mr. King himself realizes this and knows that the series is flawed. However, I'd still recommend reading all of the books. They're not the standard Stephen King horror far and he really is a good writer.
Both the fact that books are lengthening and movies are getting shorter are a reflection of power and money at play more so than anything positive or negative about people changing their desires or habits. It's rather sad on both fronts when art is distorted by market forces in this fashion but I guess its unsurprising in an age when so much energy is focused like a laser on milking every last dollar out of the consumer.