Just a bit more info, FWIW [For what it's worth]
The automatic setting of System Restore points is, as pointed out, limited by the amount of space the computer has been set for. I have seen some systems with as little as 5% allocated, some with as much as 20%.
When this protected (and hidden as 'System Volume Information' in the Explorer directory list) space gets filled up with Windows automatic Restore Points, it will deleted/remove the oldest one(s) in order to make room for the new one to be created. So it is a rolling history. It does not count days, per se, but keeps only as many of the most recent as its allocated space allows. And it will generally create one per day, or per startup. And some program updates and installs will force a Restore Point.
Re. suggestion to use ERUNT - Yes, I also recommend this, especially for Windows XP users.
But, as the ERUNT docs point out, because of differences in Vista/W7, you have to lower the UAC protection level to get ERUNT to work properly. See their FAQ for details ERUNT FAQ link.
So the end point from me is:
A) if you have enough free hard drive space, increase your Restore Point storage to some amount which gives you the length of history you wish. Make small changes only and then check. And in general, do not allocate more than 20%.
B ) Note that you should always be maintaining at least about 15% free disk space. So if your Restore Point storage is small because you have too little hard drive free space, Best to move some user data files off and create more free space.
C) note that, once the old points have rolled off, they really are gone.
D) unless you are a technically knowledgeable user, there may be value in keeping UAC at its default middle value. As turning it off defeats the protection that Microsoft believes you need. You have to make your own choice on that one.
Hope this helps. Admittedly, Restore Point discussion is a bit arcane, especially for the non technical.