Bow Length Duration on Strings
The time of how long one bow length lasts on string instruments is massively depending on the dynamic degree it is being played. For instance on Violins playing a super soft pianissimo, it is possible to extend the time until you need to change bow direction to approximately 12 seconds while it might only last half a second in super bold fortissimo. The dynamics in between are approx. seconds at piano, 3 seconds at mezzoforte and 1 second at forte. With increasing size of the instruments these lengths get gradually shorter. However, take these values with a grain of salt as forte in one piece might not be the same forte in another piece (as dynamics are never considered to be absolute but rather relative).
While these values don’t need to bother you as composer or orchestrator on long sustaining notes (as the player(s) will do more or less inaudible (and staggered) bow changes when needed), they are particularly important when writing legato parts for strings. In notation for string instruments, slurs have a different function as for instance in piano music notation (where it indicates a phrase). For strings, all notes under one slur indicate to be played with one bow stroke. Hence, a “piano like slur” as often seen written by inexperienced orchestrators would usually overstretch the possible length of one bow resulting in your players needing to work out a new bowing pattern which will cost rehearsal/session time.
So generally, when writing legato passages, keep this in mind and try to adjust your slur length accordingly. However, there are a few more factors at play when writing slurs for strings that need to be taken care of which will be covered in another post soon.