Good strings, such as d'addario and ernie ball among others, are pretty good at not breaking. The strings on my bass are roughly a year or so old and they still work just like they did, sure I should replace them and all and have a fresh set, but the point being they still haven't broken. It's good to have a backup set, just in case, though if you're working on one package, you've got time to get another pack before they break or even need to be replaced. As far as a time limit on when's best to change strings goes, that (in my opinion) depends on how much money you're making. I'm poor so I make strings last, for example keeping the same set of strings on my bass for a year.