Battery hens are those used to lay the cheap grocery store eggs. They are kept in very small cages (battery cages) in which they can't really move and just eat and pump eggs out until their egg laying slows and they are slaughtered as they've outlived their value to the commercial producer. An ex-battery hen would be one that was either rescued from the cage, or more likely rescued from slaughter. I think they usually use white leghorns for this.
Anyway, to the poster: I'd imagine you could have them as long as you like. No idea when they'll stop laying, if that's more your question.