I wonder if this is because the license really needed to be verified again or because the PS4 always tries to verify game licenses if it's connected, even if theoretically the last verification is still valid.

If it's the latter you might have just been able to disconnect the PS4 to play the game with the still valid verification from before.

But I have no clue how long a verification is valid, it might be 24h like MS wanted to implement, or even up to a year or something. Maybe it even varies from game to game.

I would guess it's random :p
Sony doesn't seem too competent/consistent when it comes to these things. According to the web portal for psn my ps+ is still expired. I never let it expire and renewed it manually a couple months ago. ps+ games on my ps3 claim their license has expired long ago yet they start without any problems, online or offline. It all works in the end but I'm afraid to learn about the horrible mess that goes on behind the scenes :)

The statistics are examples. Since the majority PC games aren't pirated to the point of the industry collapsing, then its reasonable to assume that DRM has a significant effect on preventing that. Furthermore, not only would collecting stats on the individuals pieces of software that use DRM versus those who don't, the various types of DRM within, even if reducing it to only software that could be considered games, would be a extremely difficult task that would require millions of dollars and hours of manpower for the sake of getting more accurate number for an example despite the fact that it wouldn't have any bearing on the overall topic of my post.

So dismissing these facts on the grounds, that you disagreed with the fact that I didn't waste my time researching examples, is superfluous. If you didn't bother reading why did you even bother posting?