I often get asked how often I recommend you change the strings on your guitar. I can say with some certainty that the people that ask this question aren't changing them anywhere near often enough. Now, I don't say that because I want to sell lots of packs of strings. So, how often should you change them, and why?
Well, the first part of that question I usually answer with 'If you can't remember the last time you changed them, then it's time.' (What a wit I am). The real answer is more difficult, as it does, to a certain extent, depend on how much you play. I say 'to a certain extent' because strings lose life by just being out in the open air (which is why many string companies package their strings in sealed packets). If you're playing every day for several hours, non-coated strings should really be changed every 3 or so weeks. If you're just an occasional player, you might get away with 6-8 weeks.
By far the most wearing element for strings is the sweat, dirt and skin cells from your cells (sorry if you had a mouthful of lunch when reading that). This gunk gets stuck down in the winds of the wound strings, reducing their ability to vibrate freely, which in turn makes them sound dull and lifeless. On plain strings, they cause oxidisation and in extreme circumstances, rusting. This answers the second part of the above question.
To combat this, you can clean your strings after playing, which can extend life. There are also plenty of companies making coated strings that help to eek out extra life from them.
The other part of the guitar that can suffer from old strings is the frets. Corroded strings cause extra fret wear, which, if you've ever had to pay for a fret crown or re-fret, you'll appreciate how expensive THAT can be.
So, a fresh, clear tone, reduced fret wear and a smooth playing experience - why WOULDN'T you change your strings more regularly?!