After reviewing the Battlefield 3 Beta FAQ section there will be no limitations on anything during the open beta. When we say anything we mean everything will be available to unlock.

This means you can play as long or as much as you like and rank up as high as the system allows. It also appears that all unlockables will be available during the time of the beta.

ARE THERE ANY RESTRICTIONS TO PLAY TIME IN THE BETA?
YOU CAN PLAY FOR 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.

The Battlefield 3 beta starts on Thursday, September 29th on all platforms. Owners of Medal of Honor Limited Edition and Origin Battlefield 3 pre-orders will start on Tuesday, September 27th.

Source: Battlefield.com