If you have an Electric or an Acoustic Guitar
1) Have you had the same strings on for over 3 months?
2) Are they rusty or rough?
4) Do they sound dull?
5) Had one break recently?
If you answered yes to any of the above, its probably time to replace your electric guitar strings or acoustic guitar strings.
They 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 them.
If you play a lot, say for 2 hours a day, every day, then you should look at changing them every month.
If you play less, but still strum most days, the maximum you should leave them on the guitar would be a few months.
There is an alternative to changing them this often. You can buy coated ones. These last longer, preventing dirt build up and corrosion, and can last 3 to 5 times longer than normal ones, so you can leave them on for longer. These still need to be changed though. At least every 12 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 guitar strings do cost more, about £12 for a set of 6 for electric or more for acoustic, but last longer, so can work out more cost effective.
If you have a Bass Guitar
1) Have you had the same ones on for over 6 months?
2) Are they rusty, or do they sound dull?
3) Had one break recently?
If you answered yes to any of the above, its probably time to replace your bass guitar strings.
If you play hard or play often, then change them about every 3 to 4 months. If you don't play that hard and don't play that regularly then change them about every 6 months. Some gigging bassists will put a fresh set on before every gig!
If you have a Classical Guitar
Here are a couple of questions to answer:
1) Have you had the same ones on for over 4 months?
2) Are they dirty, or do they sound dull?
3) Had one break recently?
4) Is your guitar hard to keep in tune?
If you answered yes to any of the above, its probably time to replace your classical guitar strings.
If you play often, say for 2 hours a day every day, then you should look at changing them every month. If you play less often then look at changing them every 2 to 3 months.
Under no circumstances put acoustic steel strings on a classical guitar!
Classical guitars are not designed to take the extra tension, and you could (and probably will) cause irreparable damage to the guitar.