DOTA 2’s release date has yet to be announced. As of right now the game is in beta and players can sign up for free DOTA 2 invites to gain access to the beta. The game will be free to play upon the official release. [Elder Scrolls Online is coming to PC]
There is another way to get into the DOTA 2 beta without having to wait, instead players can opt to buy early access to the game for 30 dollars at the steam store http://store.steampowered.com/app/570/. This purchase supposedly comes with some extra in game items and a free invite for friends.
DOTA 2 is likely to official release sometime later this year, or at the latest sometime next year. It would be very surprising if the game went more than two years before official being released.
Until then, try buying the early access and enjoy the game now. Supposedly the game is basically done, Valve is just working on finishing up all the minor details as of right now. Check out this sample video of DOTA 2, and check out the “suggested by the author” links below for more.