May 4th all subscribers will get 12x xp for class story lines. There will be an item to disable this if you don't want it. In addition to the increased xp the amount of commendations given by story line quests will be increased and the cost of planetary mods decreased so that it will be easier to stay relevant with gear.
There is no set end date for this increase, most people assume it will end once the new movie comes out or that it will become a permanent bonus for subscribers and this is just bioware's way of making it more exciting!
Anyway, if you want to come back this summer is the perfect time to. The game has changed a lot and is very fun if you are looking for a game to play.