Something about Legacy Skins

Before you start typing, I am not saying these are bad or that this theme of having Legacy skins only available once in a while is bad, it makes them much more worth buying after all.

What I do want to know is that for example now we have Twitch which has his Legacy skins available because he got the new splash arts. For how long are they available and when can we expect legacy skins like his as well as on other champions to become available? Obviously I don't mean stuff like skins related to LCS or skins related to holidays.