At all times. Don't ever leave home without it, Athena. I mean it.

[She's smiling, yes, but there's some serious worry in her voice. You've proven to be a little forgetful at times...]

The best place for it is on your bag, like mine. Or in your purse or something like that. It's not a strong enchantment, it has to be within a few feet of you, the recipient, for it to work. And it will only help you, so you can't rely on it to shield other people unless you give it away.