Every couple of months or so IMO. Some people say every month but i can't be bothered/don't have the money for that
Well i didn't string my acoustic for 7 monthes and it sounded shit. When i restrung it sounded so much better and was therefore a lot more fun to play.
depends on how you play and what kind of strings you get. If I had a pack of $10 martin strings and all I did way play finger style I would change them every 1-2 months. If I played with just a pick on a pair of coated strings like elixirs I'd go 3-4 months before I changed.
i got some elixir polyweb's(very expensive but worth it for sure) like a year and a half ago and i still have them on. the sound quality has not declined a lot and ive used the guitar a lot, taking it on roadtrips and performing, etc. those strings are the shit
Change em when they start going out of tune really often, or when they sound shit.
Often I only realise how shit its sounding (the guitars crap which makes it harder to tell as well) once the strings start going out of tune easily. Because it happens gradually you get accustomed to the shitness of the old strings.
But when you change them.... it's sounds awesome.
I change roughly once a month, if I'm slack every 2 months.
On this subject I need to go buy some strings.
like once a month?
when i have to mod my guitar every once in a while i switch too.
i thought ernieball 9s are supposed to last longer...

Edit: oops its an acoustic/classical thread...
sorry i change my acoustic guitar strings like once in 3 months.
so when it breaks, i don't have stock. :p
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The Elixir polywebs I had lasted me two weeks...no shit. Unless I bought a duff set I won't use them again, although I did like the tone. Gone back to my trusted EXP's...change em about once every two months to keep that crisp sound.
Hals1976 those polywebs you bought were duds, but the newer nanowebs still last like twice as long as polywebs.