I have a related question that is related to the OP question.
How far back in time does Win7 make restore points by default or at least what should the original factory setting be?
It's difficult to give a definitive answer on that one although I think the default setting is around 10% of your hard drive space, so the bigger your hard drive, the more restore points will be available.
Restore points are saved until the disk space System Restore reserves is filled up. As new restore points are created, old ones are deleted.
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