It has just been confirmed on the official SWTOR Website that Early access could be upto seven days in advance depending on when you pre ordered.

*Customers who redeem their Pre-Order Code at the Code Redemption Center prior to launch will be able to pre-load the Star Wars: The Old Republic Game Client before the official Game launch. Pre-loading the Game does not grant you access to the Game. Pre-Ordering the Game only allows you access to the Early Game Access period and your Color Crystal. Early Game Access may be up to 7 days. The length of your Early Game Access depends on the date and order in which you redeemed your Pre-Order Code. In order to continue playing the Game after the Early Game Access period, you must redeem your official Game Product Registration Code at the Code Redemption Center.