What are the signs that one needs new violin strings? I have read/heard about 1Year. This is a guess. I would like to hear something scientific!
When it don' sound good.
My luthier's rule of thumb is to change strings after 120 hours of use. However, if they are unraveling or sounding way off, you need new strings. The real problem is that the sound quality can decline so gradually that you do not notice it immediately.
Intonation goes wonky and strings don’t react as quickly and easily. But, like tires, earlier than you think is better. If you’re thinking about it, your subconscious already thinks you should.
It really depends on the brand of string. Some last longer than others. It's sort of like a hair cut. It looks fine, then it looks really good for a week, then all the sudden, you look like a wild child who's been raised by wolves. I know it's time to change my strings when they don't sound in tune, even though they are. The overtones aren't right.
I just try to change strings after about 120 hours of playing, though I've gone as long as 200 hours with one set. (I advise against trying to keep any set of strings longer than 150 hours, even with the most durable brands; if my strings stay on the instrument longer than that it's not intentional.)
Tonicas have a decent lifespan. Still should be changed every 120 hours, or at least once a year (preferably twice a year).