Increase or decrease it? Maximum retention is 90 days but restore points
are deleted on a FIFO basis when the maximum space allocated to them is
reached. By default this is 12% of the drive which is way too much. Also
System Restore is only effective going back a week or two at most. It's
best to reduce the amount of space allocated to it for that reason. The
more restore points there are the greater chance something will get
corrupted and then System Restore won't work at all. See MVP Bert Kinney's
excellent page on system restore for detailed info.
System restore is not a backup solution. It does nothing for data files.