After much discussion on the web and many misconceptions about how frequently strings should be changed, I decided that a more detailed summary is necessary.
Let’s start by mentioning that there are several factors that affect the life of your strings, not just playing time:
1. Age of strings and how they are stored
2. Length of time under stress (tuned)
3. Hours of playing – type of playing and fingers (acidic, sweaty, dry)
If you are not a professional and don’t have a special performance or audition coming up, I suggest that you change your strings when they loose strength, unravel or can’t play perfect fifths.
Full time professional violinists typically play between 30 to 60 hours a week. Most orchestras have approximately a 30 hour on stage week and its members are expected to practice on their own time. Most musicians accept gigs in their free time adding to this time. Obviously the hourly playing of professionals varies tremendously but if a violinist plays an average of 30 hrs/week (120 hrs/month), they can generally play on the same strings for at least 3 months with good results.
I regularly change my dominant strings after 6 weeks to avoid dealing with bad, flat, weak sounding, or unwrapping strings. I change them before problems arise and I have very acidic fingers. Most of my colleagues play for 3 to 6 months before changing.
Those who change their strings most often usually change after approximately 180 hours of playing, and those who pushes the limit plays 720 hours before changing their strings.
Provided the strings are fresh to begin with and you play frequently, you can expect between 180-720 hours out of your strings.
I read an amateur violinist blog where the users mentioned that strings last approximately 120 hours. One violinist mentioned that the string makers themselves refer to this figure. String makers would obviously have you think that you need to change your strings after 120 hours to sell more strings. This could be true if a violinist plays an average of 20 minutes a day, (120 hours a year). Then they may need to change their strings after a year due to the fact that the strings also have been under stress.
I have tested instruments that were newly strung years ago but have not been used for many years. If a string has been under pressure for a very long time and not used at all, it may need to be changed as well. Strings can be bad without ever having been played on.
Combine multiple factors into the equation and you will get completely different results.
Finally, all strings are not equal. From my experience steel core strings seem to last the longest but can have a harsh sound. Synthetic core strings last longer than gut strings and have a similar warmth in sound but are not suitable for all instruments.
I use Obligato, which is a synthetic core string, and am completely satisfied with the sound and life span.