Well that depends on how much playing you’re doing. If you’re playing for 20 minutes once a week you won’t need to change strings very often compared to if you’re playing 2 hours every day you might need to change strings every 2 weeks or less.
While this might seem gross another factor is our sweat. Perspiration from our fingers can contain certain elements that contribute to the corrosion of the strings. I had a work colleague and when he would put new strings on his guitar after about 10 minutes of playing they would already be starting to corrode. Even if you’re not playing and just leave the guitar lying around the strings will still need to be changed at least once a year.
Generally the best guidelines are:
1. when you notice the colour of the strings starting to look ‘dirty’ and
2. when the brightness of the sound from the strings is no longer there – then it’s time to change.
Also worth noting that worn out strings will lose their intonation (their ability to stay in tune for the full length of the guitar neck) and will probably hurt your fingers more.
New strings will go out of tune a lot for the first week or so, this is perfectly normal, the strings are being stretched under immense amounts of tension so they take a bit of time to settle down. Nylon strings will take a bit longer to settle and they will go out of tune every few minutes for the first couple of days. This is nothing to worry about and you should be tuning your guitar every time you pick it up to play. You do tune your guitar don’t you? Yes every time you pick it up you should check the tuning – what’s the point of playing out of tune?