For me, I personally adhere to eat x.1 patch being roughly 2~3 months of time passing. Reason I think this is that they always refer to you as if lots of time has passed since last seeing you. Cid calls you old friend when you show up to do Crystal Tower, even though nowadays you can do it immediately after Praetorium events within the same in-game day lol.

As for 2~3 months specfically, it's because in Heavensward when you are invited to dinner with Aymeric, the Fortemps man who passes along the message for you do so exclaims that it has been, "Several moons!" which as others have said is months.

Of course, if we wanted to get literal we could calculate it based on how long it takes in real time to complete the MSQ without skipping cutscenes. This of course, still depends on queue times for duties, but why we can still calculate it is because the in game time doesn't top ticking. One hour of real time is one day within the game, I think. Surely, someone has the exact time ratio, but we will go with that for now.

If you only do MSQs, last I heard before the 5.3 skimming of ARR, it took roughly 250 hours real time to get to the start of Shadowbringers for a newbie. That's 250 days in game time, which is about 8.5 months. Of course, if you do all side content, as encouraged by the time it takes them to develop patches, then it's easy to add another 100 or so hours, depending on a lot of stuff like queue times, PF start to fill times, and actual duty completions. Actually add in Shadowbringers, and we're likely sitting happily at about 1.5 years. Honestly, still doesn't feel like enough time passing to me, but then again, I don't even know what moons passing or years passing in game actually means for the planet we're on, since they celebrate seasonal festivals in the identical seasons to our own, despite time being massively condensed.

There's really no hard answer until they give us one, and they will not give us one, because that means they will have to change character appearances, answer more than one question about it ad nauseum, and all that jazz. I definitely agree with the waiting for the twins to age marker. Though I simultaneously find it weird that there's no sex differences between time intervals for Elezen growth spurts. You'd think Alisaie would get tall first and we'd get to see her rub it in on poor Alphi-boy.