restore points

Why do old restore points disappear? Shouldn't they stay there? Is it dependent on disk space for restore? I try to create a restore point everyday in case of something happening to computer so I can bring back system to what it was. How can I have a restore created automatically every day and me not do it?
Thanks
Windows 7
12 gig
1 TB Hard Drive
One extra Passport 1 tb hard drive

Thanks again.