If you have a Classical Guitar

For classical guitars with nylon based strings (yes 3 of the strings look like metal, but they are a thin metal wire wound around a nylon core).
Here are a couple of questions to answer:

1) Have you had the same strings on for over 4 months?
2) Are your strings dirty?
3) Do your strings sound dull?
4) Had a string break recently?
5) Are your strings hard to keep in tune?

If you answered yes to any of the above, its probably time to replace your classical guitar strings.

Because of the way classical guitar strings are made, they don't rust. Classical guitar strings age though. When they age they tend to stretch and become hard to keep in tune. They may also become brittle and prone to breaking. If you don't want to wait till this starts happening, you should look at changing them regularly.

If you play often, say for 2 hours a day every day, then you should look at changing your guitar strings 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 tension of acoustic steel strings and you could (and probably will) cause irreparable damage to the guitar.