@charlespayne exactly described by Yitra "That depends on your definition of playable."
I think you point out, when the game will have enough content to be worth to play.
I am expecting a good amount of content in Beta 2 and therefor the most possible good playable test version, since it will be the version that will be most close to the release candidate.
But you should be aware about, that even in Beta 2 we will have not all content thats planned, not even with release start
But playable in sense of enjoyable, it will hopefully be in Beta.
If you like to find Bugs and help to improve the Game Developement with constructive critism, then you will probably also have fun in Alpha 2