Do strings wear because of the pressure applied to them or any other factor, or do they stay in quite the same shape as long as you don't use them?
It depends on how you keep the guitar; the higher the humidity the more likely the strings are get get rusty and tarnish; in fact the whole guitar needs extra care in those conditions; good rule of thumb is if the guitar is not to be used keep it in a dry place at room temperature in a case if possible.
If you keep the guitar in good conditions the strings will take years to wear out if not played.
How do you know if you need new guitar strings?
For Acoustic and Electric guitars Couple of questions to answer:
1) Have you had the same strings on for over 2 months? 2) Are your strings rusty? 3) Are your strings rough? 4) Do your strings sound dull? 5) Had a string break recently?
If you answered yes to any of the above, its probably time to replace your strings.
Because of moisture on your fingers and in the atmosphere, strings corrode and rust over time and their ablility to vibrate diminishes. This not only causes the sound to dull but even worse, the feel of the strings becomes rough and will hurt or even damage your fingers.
Strings can also become brittle from too much vibration, just think of how a paper clip snaps if you twist it too much, the same happens to strings - especially if you use a lot of different tunings.
If you play a lot, say for 2 hours a day every day, then you should look at changing your guitar strings every month. If you play less, but still strum most days, the max you should leave them on the guitar would be 2 months.
Strings are quite cheap, about £5 for a set of 6.
There is an alternative to changing your strings this often. You can buy coated strings. These last longer due to a coating on the strings that prevent dirt build up and corrosion. Coated guitar strings tend to last 3 to 5 times longer than normal strings, so you can leave them on for longer. These still need to be changed though. At least every 6 months these should be changed as they are also prone to breaking because of the vibrations of the string as explained earlier using the paper clip analogy. Coated strings do cost more, about £12 for a set of 6, but last longer, so can work out more cost effective.
Ernie Ball Coated Strings have titanium reinforcement to make them stronger and less prone to break.
For Bass Guitars, the strings are much more substantial and tend to last longer and hold their tone. Change bass strings at least once a year.
For Classical Guitars with nylon based strings, again because of the way they are made, they don't rust, so last longer. They do age though, so do need replacing. When they age they tend to stretch and become hard to keep in tune. If you don't want to wait till this starts happening, you should look at changing them every 3 to 4 months.
So to summarise: Change your Electric / Acoustic uncoated strings at least every 2 months. Change your coated Electric / Acoustic strings at least every 6 months. Change your Bass guitar strings at least once a year. Change your Classical guitar strings at least every 4 months.
In my experience, old strings start to weaken at the nut or on the bridge, basically where they are stressed by pressing against the hard surface. They develop a kink at that point. This means that when you try to fine tune the string, it "jumps" suddenly to a wrong note just either side of the exact right note, and it's a killer trying to get them in tune.
And I think this happens regardless of whether you're playing it often.
This sadly happens to me quite a lot. I have hyperhydrosis so my hands sweat quite a lot and I find if I play for a bit then put my guitar away and come back to it in a couple of days it shows signs of rusting. While not everyone suffers from the same problem as me, I'm sure even a small amount of sweat from the string friction would be enough even if you don't notice.