I remember breaking my first string :) Almost hit my eye.. And I was super afraid to tell my parents.. (I thought I broke the whole instrument because I believed that the string was as much a part of the instrument as the neck or fingerboard)
I'd like to address your question by answering a few you didn't explicitly ask..
1. Why do strings break?
Obvious answer: when the string can no longer stretch to accommodate the level of tension placed on it. There are two primary factors that go into a string breaking:
- The string's tension threshold: whenever tension is applied to something, its threshold is deteriorating. The more the tension, the faster this threshold lessens (think of pulling on a laffy taffy, and how it takes less force to rip it as you stretch it out). As stringed instruments place constant tension on the strings, the strings will naturally break at a lower threshold over time. As strings initially have a higher threshold than desired (as it affects tone), the time it takes to lower it to the desired threshold is called "breaking in" the strings, which you will often see described in string catalogs.
- Tension: the string will break when it has reached the threshold described above. It theoretically can't break without some sort of tension, no matter how low the threshold (gravity is technically a source of tension, although it's quite insignificant when it comes to really small masses). In addition to physical items (pegs, bridge, etc) that provide tension, temperature plays a very important role in tension. A higher temperature causes the string to expand and the tension to lessen (as the string is technically longer), while a lower temperature causes the string to shrink and creates more tension.
2. What can I do to not break a string? The most common way to break a string is to simply turn the peg too far. To avoid this, there are two things you can do:
- Tune down first when using the pegs: Tension often builds up based on various factors including changes in temperature. It's very possible that you're right at the threshold, and the string snaps as soon as you turn the peg even slightly higher. By tuning down first, you 'reset' the tension a bit and get a better feel for both the amount of tension on the string and how close you are to the threshold.
- Use fine tuners: if you don't already have fine tuners, I'd highly recommend you get them. Whenever you're not too far away from the correct pitch, fine tuners make it easy to get a more precise pitch and reduces the risk of overshooting and breaking the string.
3. How do I know when to change strings? Using an objective method, the only two ways I can think of for knowing when to change a string is either when you can see the string wearing translated as sharp/rough spots on the string, or after you've already broken the string. But when we get into the more subjective area, there are a few ways, all which come with time and experience.
There are essentially two ways to tell that you need/want to change strings: the tension feels weird, and the strings don't provide the sound you want (anymore, or they never did in which case you'll want different strings anyways).
For both of these, you'll need to be in a place where enough of your playing has become second nature, and you're exploring more advanced stuff.
In order to be able to tell when the tension is wrong, you need to have been playing for long enough where it's just something you notice. It'll be most notable (in your left hand) when shifting and crossing strings.
Responsiveness and tonal quality are things that require you to know the sound of a violin and specifically your instrument very intimately. The responsiveness is essentially how easily the string responds to your bow. This is a quality that varies by strings and deteriorates over time. When the strings are not responding the way you want them to (and you are in a place where you know what responsiveness you want/how to achieve it), then it is time to change strings.
Tonal quality/timbre is generally measured for violins in spectrum that ranges from warm/rich (produced more when strings are wound with gold) to bright/clear (produced more when strings are wound with silver). The tonal quality varies from instrument to instrument, and also from string to string. This is why it requires you to both know your instrument very well, and then also to know/have experimented with different strings. In retrospect, I don't think I could was even in a place to consider these until I had been playing for 7 years, with four on the same instrument. A few things to take into consideration when looking at tone are the instrument (wood/varnish/shape), bow(wood/shape/hair), and string (core/wound/gauge/tension). Essentially, when you stop getting the tone/response you want from your strings (and it's not your fault), then it's time to change strings.
Let me know if you have questions about anything I said :)