A: How much do you play? And what kind of strings are they? What I tell most guitar players is that they should change strings a minimum of once every 2-3 months. If you are a heavy player you may want to change them more frequently. Some players can make strings last for years. This is not recommended as strings lose tension as they get older and this allows the wood in the neck to warp more easily.
Bass players should change strings every 5-6 months minimum. For those players who boil their bass strings, this time can generally be stretched to 8 or 9 months though boiling strings is not recommended for optimum long-term maintenance of the bass. Note that not all bass strings should be boiled (e.g. Elixer).
NEW STRINGS JUST SOUND BETTER! So change them as often as you can. It will help keep your guitar in optimum condition for a longer period of time and you'll be more content with your overall sound.
Oh, and if you bought your strings from Target, you should probably skip putting them on your guitar and put them directly into the trash where they belong.