This may step on some sensitive toes, but for a long time I have believed that the law of diminishing returns comes into play at some point, probably sooner rather than later, in all talk therapy. No, I'm not an expert, just a guy with an opinion formed by observation and life experience. However, I do happen to live in a place many people consider therapy central. That is, I live in the San Francisco Bay Area. If nothing else, that should give me some measure of "street-cred" when it comes to the subject.
It appears my suspicions are at least partly confirmed by a report in Scientific American. You can read about their findings here or listen to a short podcast.