The frequency with which you change your guitar strings will vary with your playing style, the cleanliness of your hands during a playing session, how well and how regularly you clean your strings etc etc so there are no hard and fast rules. For sure if your strings start to look discoloured or sound "dull" a change is due, but for most non-professional players - and using normal strings - they should give you good service for between 24 and 36 hours of playing time. Strings with special coatings can extend this time considerably, although you still need to remember to play with clean hands and to wipe your strings and fretboard after each session if you want to get the most from them.