That depends. How often do you play? If you're playing every day for at least a half hour, then you'll find that the tone of your strings will deaden after three or four weeks. Maybe sooner, maybe longer. If you play a little less than that, or if you're just not sure, then it's a safe bet to have them changed every other month. You can bring your instrument in to a guitar shop to be re-strung, or you can learn how to do it yourself. It's really not that hard.