Depends entirely on the instrument.
Modern standard P/J I'll change in the studio every 3-5 days of use. Live, since I sweat more, they get changed daily.
I do have problems with pitch sometimes with old strings, but I try to work around those problems, because I'll be damned if I'm changing them.
Flats: I don't change until they break, unless the tone just becomes far too lifeless and the bottom end begins to fade, which I have noticed.
Jerose: "Don't forget LEDs!...you need enough to effectively render an assailant blind...once he's defeated you can reward yourself with Pez".