Hi,
First of all System Restore is not a backup tool.
By default System Restore automatically creates a restore point every 24
hours. That is as long as there's enough system idle time for it to do
so. Restore points may also be created before an application
installation, installing Windows updates, and one is created when a
restore is performed to make an Undo possible.
Restore points by default are automatically deleted when they become 90
days old. Or if the amount of disk space used to hold the restore points
reaches it's limit. As already suggested, it is not wise to delete all
restore points. A better approach is to limit the amount of disk space
used to hold the restore points.
Here are some tips for adjusting disk space usage and keeping System
Restore Healthy
http://bertk.mvps.org/html/healthy.html#AdjustDiskSpace
And here's a description of System Restore:
http://bertk.mvps.org/html/description.html