Okay, there's more to it than that. Here are a few pointers I've learned about string:
·For classical, nylon-string guitarists, the bass strings go out way before the trebles, so the 4th, 5th & 6th strings should be changed every month or two more often if you play a lot. You can leave the trebles on for months. They don't corrode and degrade as fast as the basses. Every 3rd set of basses, change the trebles.
·For electric and acoustic steel-string guitarists, ignore all that. Change the whole set.
·For any style player, a fresh set of strings will make your guitar live up to its potential and make your playing more enjoyable, both for you and your audience.
·Classical and other nylon-string guitarists: don't change all of your strings at once if you have to perform within three days. Try to always have three strings that have fully stretched out as you break in the other three.
·It's better to more frequently change low- and moderate-cost strings than to hang on to high priced strings past their prime.
·It's better to perform with dull strings that are in tune than brand new strings that sound bright and new but can't stay in tune during your performance.
Here's a link to a webpage with detailed instructions on how to change