Newly released skin available from hextech boxes?

One of my friends told me that newly released skins have a month long cooldown period before showing up in hextech boxes,but I could not find any official sources to confirm that. Right now I have 3 boxes waiting to be opened, and I wonder wether to not wait after the star guardian skins are released to take a shot of getting them, or wait an extra month after that. Does anyone know the details on how new skins are treated in the hextech crafting system?