Assuming you are a guitar beginner, how often to change guitar strings depends on how often you play it. If Beginner A practice guitar for 30 minutes per day, and Beginner B strums 2 hours daily, which player do you think would need to change guitar strings more often?
A good rule of thumb is when your strings started to sound dead, or when a string (especially a lower string) breaks for no apparent reason, then it is time to restring your guitar.
In general, change your guitar strings every 3 – 6 months should be a good idea. See our another article on How to Restring Acoustic Guitar.