Just to quote from swtor.com
One of the benefits to pre-ordering Star Wars™: The Old Republic™ is that you will gain Early Game Access before launch on December 20th. This is a chance to start your epic Star Wars saga early, as you’ll be able to create and play your character ahead of the official live game launch.
Many of you have been wondering how long Early Game Access will last, and we are now happy to announce that depending on when you redeem your Pre-Order Code, you will gain Early Game Access up to five days before the official game launch date.