This is not meant to be a post complaining about the difficulty of actually catching the fish but rather the availability of opportunities to even try.
So far, this has definitely been the rarest of the Legendary Fish added for Stormblood, with windows commonly reaching gaps of 6-7 days. These windows are rough but nothing we haven't seen before, dating back to ARR.
That being said, in the next 30 days there are going to be a total of two opportunities to catch The Ruby Dragon, including a 23 day gap between May 22nd and June 14th.
I can't help but feel this window is an oversight of what is reasonably attainable by the player base considering that all available windows (for any fish, not just this one) cannot be expected to be available to the average player. No other fish in the game, to my knowledge, has this level of rarity for an opportunity to catch it and that tells me that it's a massive outlier that doesn't fit with the rest of the fishing content that's available.
Additionally, in the case of The Ruby Dragon, this fish was released close to the end of this expansion, but with this kind of rarity the opportunities to even catch it before the next expansion have been unreasonably thin.
Perhaps the 23 day gap coming up that is the true outlier, but if such an outlier is even possible for a weather pattern then I would say that no fish should be tied to it unless its actual catch difficulty is as simple as the Ku'er needed to catch the Legendary in question.