Years ago, when I played long hours outdoors for limited money (as a streetsinger), I played until I broke a string, then replaced the broken string with a used one until I bought a new set. Soon (the next day, usually), I'd replace the whole set, saving the five unbroken strings for the next emergency.

Since the first string to break could be any of the three or four highest-pitched stings, this system usually worked. (If it was always the same string that broke, of course, I'd never have the necessary used-string replacement.)

I'm still in the habit of keeping 10-12 old strings in my guitar case, even though I no longer play such long hours ~ and nowadays play much less often in public, and then strictly indoors and through a microphone. I now usually change strings when I notice a deterioration in sound, and don't wait until one breaks. I don't mark the calendar and don't know how often the mood strikes me to update the strings, and I certainly don't know how often I "should" change them. I do know that my involvement in playing ebbs and flows as other aspects of my life demand more and less attention, and the frequency of my string-changing follows suit.

A few weeks ago, I actually broke a string for the first time in years: I had a used replacement string for immediate use, and when I bought a new full set later that day and put 'em on, I noticed a remarkable improvement in tone. I had left the previous set in place for much too long, thanks to more-complicated-then-usual developments in "real" life.

I use medium-weight phosphor bronze strings ~ 13s ~ on my all-acoustic guitar, and use a plastic thumbpick and two steel fingerpicks. As a street performer, I always whaled on the guitar pretty hard with those picks, which undoubtedly led to fairly frequent string breakage. I even tried heavy-weight strings once in a while to prolong string life, but found the mediums preferable for tone and even for volume.

More recently, I still use the picks for performance, although I'm sure that my touch has become quite a bit lighter. At home, I'll fingerpick barehanded sometimes (especially when I'm supposed to be relatively quiet, like after bedtime) and use the picks maybe half the time or less. I'm sure this allows my strings not only to remain intact longer than in my busking days, but to retain their tonal qualities a bit longer, too.