Depending on how far out your trip is, you always have a good chance of your waitlist working for you.
There are two types of waitlist notifications. One: you can choose to have them call you when your waitlist becomes available. By choosing this option, MS will leave you a message (if you've missed the call) and you must call them back within 24 hours or you loose your waitlist. Two: you can choose to have them automatically accept your waitlist when it becomes available, you will receive a new confirmation in the mail.
I always choose to be called. I just like knowing as soon as it becomes available.
I believe your request stays on the waitlist until 30 days prior to your trip, and if you want to stay on the waitlist, you then need to call MS back to notify them.