To get an exact overview of when the Beta is coming then check out the cool BF3 Countdown Timer
Codes to participate in both the closed and open Beta will be e-mailed out to the e-mail you used to register your Origin account.
You get the early access beta automatically in the playstation network store once it’s available.
You will receive a code via e-mail with instructions on how to download the Beta to your console.
MoH LE Steam User’s
To view your Key, go to Medal of Honor in Your Steam Library, and click the “CD Key” Link. Find “Battlefield 3 open Beta Early Access” key. Download Origin, redeem this key on Origin and start downloading the Beta.
More on the Rush Gamemode can be found Here
According to the official FAQ:
You can play as long as you want during the time of the Beta. There will be no cap on how far you can rank up or what you can unlock.
Uses Ingame Serverbrowser, also shows Battlefeed Ingame (Battlelog Status Updates)
approximately 2-3 GB
In the mean time feal free to drop by the discussion threads: