interesting question.
I don't know after how many cycles one "should" change it.
I would get a new one whenever the old one doesn't get you enough hours anymore for your work.
I mean, if your battery lasts for only one hour and that is just enough for your train trip to work, then that's alright, isn't it? Otherwise get a new one to get back to your original amount of hours.
I've read stories where people changed their batteries every year and I know people who change them all 3 years (or better yet get a new notebook by then)