I use them for practice, usually out in the desert. Then I toss the empty can into the trash.
<edit> It recently came to light that although my wife has carried a can for years, it was the same can - and she's never actually used pepper spray at all. I bought her a replacement and made her "shoot" at random targets with the old one. She was pretty surprised at how weak the spray was (it was a typical pattern; not the result of a worn-out can) so it was definitely a worthwhile exercise.