The default live time for a Restore Point before it is automatically deleted is 90 days. This can be changed as shown below. If System Restore has to delete a Restore Point because of hard drive space issues it is always done on a FIFO basis.

HKEY_LOCAL_MACHINE\Software\Microsoft\WindowsNT\CurrentVersion\SystemRestore

RPLifeInterval

This value specifies, in seconds, the restore points Time to Live (TTL). When a restore point reaches this time and it is still on the system, it gets deleted. The default value is (7776000), which will be 90 days.
The Registry Keys and Values for the System Restore Utility