I find you get a brighter sound with fresh strings rather than a bit of a dead/dull sound with old strings but it really is up to you.
If your happy with the sound of your older strings, good for you. When you feel its a bit dull change them and its all fresh again.
I cant remember in which order (or if it was just another rock star talking out his ass) but I once read Eddie Van Halen finds the oldest strings he can for recording but for playing live likes new strings, so go figure! (as i said that could be back to front - goijng on memory here)