By default, Windows 7 creates one restore point every seven days or before significant system events such as a program or driver
installation, some Windows Updates, etc. System Restore Point - Create
It also depends on how much disk space you have set aside as to how many individual restore points will be saved. System Protection - Change Disk Space Usage
Since restore points serve a very limited function, I usually keep 3-4 at any given time. I rely more on system images to return my computer to a working condition. So depending on how large your hard drive is and how many restore points you feel comfortable keeping, this is a purely personal decision.