I think it'll be around November. Until then, you're able to level up to 15 for free, so if nothing else get to work on that! Then when true F2P comes, you'll be able to level to 50.
I will say though, your options will be very limited. Some examples: you will be limited in the amounts of flashpoints and warzones you can do per week, limited in how many things you can post on the GTN, and you will not be able to raid at all. More details can be found
here.
As you can see, it's actually to your advantage to start as a free player leveling to 15
before the launch of F2P. since you'll get a few more perks. My understanding is that the cartel coins will enable you to purchase certain upgrades within the game.