Riot, when can i expect urf to be released again?

I missed urf the last time it came out and i always come back to league to play it. i have so much fun when i play urf and i dont want to miss it again so i was just wondering can you give me a time period when you might release urf next? everytime urf comes out i pull an all nighter, its just so addicting and i never get tired of it :D