For me, there are three stages in the life of my DR's...sounds kind of weird, I know. But, I use these stages to determine when to restring.
Stage one is when the strings are fresh and pick up the littlest nuances of my playing. The strings have an added kick in the attach that just sounds so cool it sends shivers up and down my spine. That only lasts a couple of hours worth of playing, which is when some people consider strings dead, such as John Entwistle.
The sencond stage is when the initial brightness wears off, there's a certain warmth that just sings in the midrange section, which gives my Spector a certain character that I love. The brightness has faded a bit by then, but the strings are just so warm sounding at this point...
The third stage comes usually a few weeks or even a month after the second stage when the strings have lost their life and just seem flat.
I generally change strings a lot less often when I'm not in a band situation, due to issues of cost and what sounds good at home. In a band situation, though, I always change strings about 3 hours before a show.