So dont be too afraid of your new strings going off a bit. How long you can keep those on then depends on you. How much you play etc. I find strings last some months on my guitars, but then they dont get played all the time...
Some players just murder strings. Their hands leak evil acid that causes the string to literally corrode right in front of you. The strings go black and gummy. We all know these players. Some of the best session players I know are string killers. You might think this would be a magic bullet in the fight against new string shiny syndrome. Sadly not. When the string killer strikes the string does not 'age' it 'dies'. That's why (s)he is called the string 'killer'. The worst possible scenario is one where you have just got through the 2-3 weeks of shiny period, the guitar is starting to sound thick and solid with chunky attack and no cluttery clattering...When in comes the string killer. Within an afternoon, the carefully matured super slinkys are gooper mankys. There's no resurrection available. People bang on about cloths and cleaners, but really, what good is cleansing oinment when you're already dead?
<hums the last post>
Sorry what was the question again? Oh yeah, just change them when the intonation starts to go off, or when you have a gig coming up and you're worried that you might break one.