How long does violin rosin last?

Answer 1

In my experience, rosin (for any instrument) has a shorter life than the manufacturers claim, because, long before the rosin goes bad, it gets dropped.

Most of the rest of what is said about rosin is unsupportable. Since rosin makers rarely reveal all the additives and the purity of the colophon (the resin that is extracted from sap, which gives rosin its characteristic stickiness), it is a shot in the dark to make comparisons. In fact, for the manufactures of the high-volume lines, there isn't even a lot of guarantee that the rosin of this month is the same as next month's batch.

Some manufacturers advertise that their rosin is made only of purified colophon, and some of it might just be. Others advertise additives which are supposed to make the rosin cause a "smoother sound", whatever that might mean. Some of these component are surprising, others are downright startling: Beeswax, for instance, melts at temperatures which are compatible with rosin-melting temperatures, and might just form a uniform admixture which is preserved through pouring, cooling, shipping, sitting on shelves, being rubbed on bow hairs, and then on strings. Others, like gold, silver and ground meteor bits (which are never qualified, let alone quantified) are supposed to have nearly magical effects on sound.

It is said that La Scala's pit orchestra used the same block of rosin for decades, and that it only required a single stroke of a bow before a performance for the most perfect sound of Violin through string bass. The recipe for that rosin is, of course, lost to history.

If you are finding that the effect of your rosin is changing within short periods (one or two months), it is a good idea to look to the amount of rosin you apply to your bow, how (or if) you clean your strings between sessions, and whether you are inadvertently contaminating the strings with oils from the skin of your fingers or other body-oils. Only after ensuring that you aren't causing the problem, should you worry about the rosin: are you using too much? Too little? Does your rosin produce a great deal of dust? Maybe one of the "professional" grades which are touted to produce little or no dust would work better for you. (These are sometimes sold as "non-allergenic", because some people become allergic to rosin dust and need a rosin which doesn't produce dust.)

If you loosen your bow and run a _very_ clean comb which has not been used on human or animal hair through the hairs, you can see if you are using too much rosin or too soft rosin for your environment fairly easily: the hairs will not separate easily. If the comb becomes covered with rosin dust, you should be cutting back on application of rosin. If the hairs appear more yellowed in the area near where you hold your bow, you may be oiling them with finger oils.

Answer 2

Manufacturers will say four years, but many violinists say one year is about as long as it stays good.
It really is a personal preference, if you notice less bite than you are used to change it out, its not that expensive and you need to be comfortable.
There are factors that can degrade the rosin faster, heat, humidity can speed the decomposition of the rosin so be aware of that.