From the FAQ here
Q: How long will opt-in signups be available?
A: Players will be able to opt in until the beta closes.
Beta will run for as long as we feel it is needed and notification is generally given when we are ready to close it down and release.
The WotLK Beta Opt-in doesn't close until the beta does. People will know if they've made it in or not by whether or not they've received an invitation and been able to download and play the beta. (src)

We will do our best to keep the community informed on the start of beta. I couldn't give you specifics on our plans at this time. (src)

Opt-in only puts you into a pool of people that we will be randomly selecting from and then inviting. You should not expect a "sorry you aren't in this phase of beta" email if that's what you are asking. Doing so would mean you're completely out of the selection pool and we fully intend to invite more people as we go through the process (as we need them). I can't predict for you how many people we will be inviting per phase nor can I tell you what the possibility of you being invited will be.
Aside from the Opt-in, I'm sure you can try your chances at any fansite run contests that may run during the time of the beta. (src)

Once you're in, you're in. No one is going to come to your house, tap you on the shoulder and tell you, "It's your time." Though that would be kind of neat if we had someone that did that. (src)