With a three to five month beta window, that puts the end of the test somewhere between May and July. Of course, there's no telling how soon the game will release after the close of the beta, but hopefully it won't take too long. The universe won't save itself, after all.
Finally, for those out there lucky enough to be in the beta, you should know that the first patch is now available. You can check out the list of changes on the Battle.net forums.
[Via Big Download]