When does the gears of war 3 beta end

After four weeks of executions performed, medals awarded, skins unlocked, and enemies decimated, our time together in the Gears 3 Beta is coming to an end. The servers go dark at 2:00 AM PDT on Monday, May 16, after which you will be unable to connect to multiplayer matches. If you want to hang on to any of your retail unlocks earned during the Beta, please do not delete your Gears 3 Beta saved games-you can delete the Beta executable itself, but hang on to those saves! (Step-by-step details below.) Thanks again for playing, and we'll see you again on September 20!

Source: The Gears Website