That's like asking "how long does it take to redecorate a house once you've decided you don't like how it looks" or "how long is a piece of rope?" There are no answers to these kinds of questions because they are different in every situation.
I always understood it to be around 30 dayish. It cant be open ended, otherwise the game would never come out. Thats what updates and such are for. I was unfortunate not to play the Aplha, but i could deal fine with a few bugs and balance issues to have something in my hands.. A delay would be ok with me, but give us the beta to play the shit out off.. Seems to me that, that would allow for a better game on launch. No?