How long are the PBE cycles?

It has been two months since I signed up for the PBE I think and l would like to know how long are the cycles for the PBE are and what are the chances of me getting in? I'm asking this because I really want to get into the PBE because I enjoy helping to make sure content is perfect when it is released to the public. I have tested many game like Elder Scrolls Online, Destiny, Star Wars: The Old Republic, FFXIV: A Realm Reborn, and Titanfall to name a few. I would like a response from a member of Riot or someone that has knowledge on the mater.