'Yen' is an elvish word meaning 'long year', but it actually represents 144 years. It appears in Galadriel's "Namárië" song, so of course Tolkien commented on it. The plural 'yeni' means countless years.
Yeni onotime ve ramar aldaron = Long years numberless as the wings of trees (wings of trees = leaves)
The significance of 144 is that elves use a base 12 number system, so that 144 is (essentially) a century. So, FA 144, 288, and 432 would all be 'centennial' years for the Noldor. Since our finale happens in FA 260, the only real opportunity to introduce the concept of 'yen' this season would be year 144. OR, to have Turgon desire to complete Gondolin 144 years after he started it (or after his dream).
Now, Tolkien had Ulmo's visions come to Finrod and Turgon a little earlier (around FA 52, before the Dagor Aglareb), and we are doing it a little later (ca FA 60-something...after the battle). But Tolkien did put a gap of about 12 years between Turgon having the vision and starting construction of Gondolin. And then it takes 52 years to build (completion date FA 116).
So, if we have Turgon have the vision in FA 62, and then start construction of Gondolin 12 years later in FA 74, and then have him complete it in 144 years (rather than 52), we're looking at a completion date of FA 218, which could be the lead up to our finale (or near enough, anyway).
We will need Gondolin to take longer than Nargothrond. Turgon doesn't have any dwarven helpers, and he's doing everything in secret. We don't necessarily need to call attention to how long it's been, but...it's been awhile.
My main concern is that it doesn't look to the audience that the elves built cities overnight or in a single year, since all of the other episodes to this point in the season are running on a different timescale. I want to convey that a significant amount of time has passed, but it doesn't have to be so blatant as Turgon announcing how many years it took to build Gondolin. Though...he could.