The beta lasts until March, or so I was told when I downloaded it. When the date hits, your computer supposedly starts rebooting every 2 hours to interrupt you. I'm not sure of the exact date, but it's march 2010.
The beta version is supposed to last until the first service pack for 7. I believe that is going to be sometime in March. If you are concerned about it get a copy of 7 when it is on sale at newegg or tigerdirect. Just when it tells you do a fresh backup and load on the full version.
You must have the release candidate (RC). Not sure the exact day but you are correct with the month. I would guess that it will be the first day of the month.