How long does Wal-Mart save their security camera footage for?

To be specific, I'm in California, but is there a general policy anyone knows about?

This isn't as shady as it sounds, I promise!

Thanks!
Update: Oh, and p.s darlings:
No, I didn't steal, I'm writing an ethnography on coporate policies, and (of course) Wal-Mart wouldn't say a word themselves. I'm just getting a good ballpark to compare with other "good ol' chains."
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