The answer depends upon a number of factors;
A What type of string instrument is involved ?
B How old are the current strings ?
C How often do you play the instrument , daily or weekly ?
As a general rule violin and banjo are loud instruments and the strings might be changed every 2/3 months , depending on B&C above !Mandolins,mandolas,guitars, bouzouikis need to have the strings changed more frequently, some professional musicians are of the opinion that one shuld replace strings daily,i remember witnessing the folk group ‘clannad’ change strings on all the acoustic instruments at the interval, thus ensuring that they had a bright ,mellifluous sound for the second half of the performance !
In these recessionary times people are searching for ways to extend the playing life of strings and are increasingly purchasing a number of different products, primarily- ‘Fast Fret’!This product does”exactly what it says on the tin “; A cleans strings
B prolongs fingerboard life
C brightens sound
C lets you play faster and won’t soil or stain the fingerboard .
Having used the product for over 20 years , i can verify the authenticity of the above claims !The poduct is available at mcquaids music shop and is relatively inexpensive , retailing at 10 euro a tin + p.&P. for mail order customers !