One fast way to see just how many and what older restore points are still available is to start up the System Restore feature and check off the "show more points" option. As you can see from the imade below the 5/30 point was still availble here while this will vary on each system.
As for the differing expiration dates the present form of the SP1 beta now seen is the developers version not having reached the one selected for general release for home users which will see a set date for expiration on that. The SP1 is still in the development stages and could be seeing daily/weekly changes.
A 4th step(not mentioned and for advanced users) is for those who created a full system image right before installation of the beta version of the service pack. Backup Complete Computer - Create an Image Backup
This would a last resort option however not preserving any new files or programs added since the image was first seen.