For instance, I just looked up Skyrim's main quest, Bing says it takes "about 30 hours." How do they figure this out? Do a bunch of devs and/or game testers do some speed runs, only focusing on the Main Quest?

That is my guess.

I doubt there was any deliberate timing done. Since your saves have the time played as an attribute you can check, I'd imagine it was a case of looking back at some saves to see how much time had passed.

So I tried to find some among my own characters, and it appears that I always saved after getting back from Sovngarde, as none of them had a save there, even though I know who did the MQ. So I had to use a different method.

Since Dragonborn assumes you did the MQ, and everything on Solstheim starts at about level 20, the nearest estimate I got was to find saves on Solstheim from level 20+ characters, and see how many hours in those were. 30 is not a bad estimate. as you need to do side-quests for XP, or you won't have the skills to complete the MQ, so a speed run won't work very well.

That's a good way to figure, ghast, assuming you only focus on the Main Quest. My problem is I'm always straying!

But yeah, I guess "speed run" is a bad couple of words to use, because I've seen accounts in which gamers boast they've "beat" the Main Quest in Three hours or whatever. I am guessing that 30 hours includes a more relaxed pace, maybe some light use of fast-traveling via the carriage system, rather than FTing Oblivion-style.