Hello Luca!

Besides upon breaking a string, when to change strings depends very much on personal taste and budget, and how often to change them is very dependent on many factors:[br][br]Do you use nylon or metal strings?

Do you use anti-corrosion coated strings (for metal strings)?

Do you play often, for long times? Do you sweat much? Do you live in a humid place?

Do you clean strings after you end playing for the day?

Do you use string care products (like fastfret, etc)?

I only play electric, so I cannot help on classic/spanish/flamenco guitars. In my case, I use anti-corrosion strings and change them when I feel them sharp (specially when doing bends) and I start seeing rust spots (usually close to frets). So I guess I change them more for the sake of playability rather than for tone. I leave that for better players than me...