Legacy Skin Question

So of course I know legacy skins are only made a available for a limited time whether it be in a bundle or made available during or because of an event, but how often do general legacy skins pop up in the store after being taken down. Im asking this because earlier this year a legacy skin I was interested in was released but I didnt have a chance to buy it due to it being taken out of the store. Is there a specific timeframe between the release dates for the skin, or do they just come out when Riot thinks the time is right?