OK, first, they were promised (there is that word again) as absolutely, 100%, a fall back thing. Something that would make DMs lives easier. The famous example was "what if the pcs decide to go deal with the mayor" and the DM hasn't thought this through at all. That is still they way they are presented in the DMG. Here are some examples, these skills work, these don't...run as is.
Second, regarding scale and time...the idea of using skills in an ongoing investigation across the adventure, is just an example of using skills across the adventure. There is nothing new there. I guess my problem with the answers above like is: what is the point? Why a "skill challenge"? I can easily come up with lots of skill uses over several sessions, and the pcs will come with more. This comes from having skills. What does a "skill challenge" add? (and, if this is about a sequance of challenges, ok, but mearls doesn't actually put it like that, and again, where are the specific "challenges").