The thing is, as long as you start it before the challenge window ends, you can complete it at any time. I have a dedicated save for challenges. I did the finding love after a break up one to test out challenges because it seemed faster than the get 1000000 simoleans one, and then I've set up households for both too many toddlers and perfectly well balanced, but I haven't actually played through them yet, and the only reason I plan to play through the holiday one is because I have a story idea that will work with it.
In part, I'm "acid testing" how long the scenarios can be "on hold" for, but because they can be on hold, there isn't actually that much pressure to play through a challenge, so I don't think they're collecting data on that. or at least that's not the focus of their data collection.
