The answer to this question is tricky, because there are so many factors which affect the life of your guitar strings. For example:
- The type of strings you use
- Your style of play
- Frequency of practice
- The cleanliness of your hands
- The composition of sweat and oils occuring natuarally on your hands
- The type of guitar (acoustic or electric)
As a general rule, a guitar string won't start losing its tonality until after about 12 hours of play. That means if you practice 30 minutes a day, you should change your strings a minimum of once per month as a start. Depending on the factors I listed above, you'll want to tweak your string changing schedule. As an example, let's say you change your strings every month, but then you break a string before the next scheduled string change. In that case, I would just go ahead and change the entire set. If this is a regular occurance (like every third week), then you'll want to make sure you change them at the end of the second week.
Sometimes extreme enviromental conditions (as well as just normal play) will cause your strings to corrode. They will become rusty or dirty over time anyway if you let them go long enough. If that happens before your next scheduled change, once again, just go on and fit your axe with a new set. I need to insert here that I never allow my own strings to reach this pitiful state. As a professional instructor, I need to change my strings weekly to keep my guitars at the peak of their sonic/tonal capabilities. When I played professionally, I changed my strings once per week when we weren't gigging. However, when we were playing out a lot, I would change before every show to ensure the best possible tone and decrease the liklihood of breakage during a live performance.