There isn't a single standard, it's up to the author(s). Similarly it's up to the author(s) when they feel something is ready for a particular stage. As for alpha and beta, here's the page in Wikipedia about it, though it is not specific to open source, and the page is flagged as needing work.
Software release life cycle - Wikipedia, the free encyclopedia
There's also a link from Wikipedia to this segment of a document:
Maintaining a Project: Interacting with Users