It depends on what you're using them for.
Live... I'd change em every night cause you never know what the previous nights playing has done. Studio, it depends on whether you're recording scratch tracks or 'performance level'. You can use the same set of strings for almost a year if you want but when you go to record them, get a new set so you get the best sound you possibly can.
(the exception being if dull sounding strings are part of your sound... there's been many times when I've loaded up a new set of strings and been disappointed because the lead I was playing was based on an old set of strings)